QST: I get an error when use to_sql func to save a dataframe to mysql for BLOB column.how to fix it ?
my package list is:
Package                           Version            Note: you may need to restart the kernel to use updated packages.

--------------------------------- -------------------
-cikit-learn                      0.22.2.post1       
asn1crypto                        0.24.0             
attrs                             19.3.0             
automan                           1.0.0              
autopep8                          1.5.2              
backcall                          0.1.0              
bcrypt                            3.1.4              
bitarray                          1.2.1              
bleach                            3.1.4              
cacheout                          0.11.1             
certifi                           2018.11.29         
cffi                              1.12.2             
chardet                           3.0.4              
Click                             7.0                
cloudpickle                       1.4.1              
colorama                          0.4.3              
configobj                         5.0.6              
cryptography                      2.3.1              
cycler                            0.10.0             
Cython                            0.29.19            
DBUtils                           1.3                
decorator                         4.4.2              
defusedxml                        0.6.0              
entrypoints                       0.3                
et-xmlfile                        1.0.1              
Flask                             1.0.2              
future                            0.18.2             
hyperopt                          0.2.3              
idna                              2.6                
imbalanced-learn                  0.6.2              
imblearn                          0.0                
importlib-metadata                1.5.0              
impyla                            0.14.0             
ipykernel                         5.1.4              
ipython                           7.13.0             
ipython-genutils                  0.2.0              
itsdangerous                      1.1.0              
jdcal                             1.4.1              
jedi                              0.16.0             
Jinja2                            2.10               
joblib                            0.14.1             
jsonschema                        3.2.0              
jupyter-client                    6.1.2              
jupyter-contrib-core              0.3.3              
jupyter-contrib-nbextensions      0.5.1              
jupyter-core                      4.6.3              
jupyter-highlight-selected-word   0.2.0              
jupyter-latex-envs                1.4.6              
jupyter-nbextensions-configurator 0.4.1              
kafka-python                      1.3.5              
kiwisolver                        1.2.0              
lxml                              4.5.0              
MarkupSafe                        1.1.1              
matplotlib                        3.2.1              
mistune                           0.8.4              
mysqlclient                       1.4.6              
nbconvert                         5.6.1              
nbformat                          5.0.4              
networkx                          2.2                
notebook                          6.0.3              
numpy                             1.18.2             
opencv-python                     4.2.0.34           
openpyxl                          3.0.3              
pandas                            0.25.3             
pandocfilters                     1.4.2              
paramiko                          2.4.2              
parso                             0.6.2              
patsy                             0.5.1              
pickleshare                       0.7.5              
Pillow                            7.1.2              
pip                               20.0.2             
ply                               3.11               
prometheus-client                 0.7.1              
prompt-toolkit                    3.0.4              
pyasn1                            0.4.5              
pycodestyle                       2.5.0              
pycparser                         2.19               
pydotplus                         2.0.2              
Pygments                          2.6.1              
PyHDFS                            0.2.1              
PyMySQL                           0.9.2              
PyNaCl                            1.3.0              
pyparsing                         2.4.7              
pyrsistent                        0.16.0             
python-dateutil                   2.8.1              
pytz                              2018.9             
pywin32                           227                
pywinpty                          0.5.7              
PyYAML                            5.3.1              
pyzmq                             18.1.1             
redis                             3.2.1              
requests                          2.18.4             
sasl                              0.2.1              
scikit-learn                      0.19.1             
scipy                             1.4.1              
seaborn                           0.10.0             
Send2Trash                        1.5.0              
setuptools                        46.1.3.post20200330
simplejson                        3.16.0             
six                               1.12.0             
sklearn-pandas                    1.8.0              
sklearn2pmml                      0.49.0             
SQLAlchemy                        1.3.15             
statsmodels                       0.11.1             
terminado                         0.8.3              
testpath                          0.4.4              
thrift                            0.10.0             
thrift-sasl                       0.2.1              
thriftpy                          0.3.9              
tornado                           6.0.4              
tqdm                              4.46.1             
traitlets                         4.3.3              
urllib3                           1.22               
wcwidth                           0.1.9              
webencodings                      0.5.1              
Werkzeug                          0.14.1             
wheel                             0.34.2             
wincertstore                      0.2                
xgboost                           0.6                
xlrd                              1.2.0              
XlsxWriter                        1.2.9              
zipp                              2.2.0

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
# Your code here

from sqlalchemy.types import VARCHAR, Float, Integer, Date, Numeric,BLOB,CLOB
from sqlalchemy import create_engine
# db_conn_str = 'mysql+pymysql://user:pswd@localhost:3307/xxx?charset=utf8'
con=create_engine(db_conn_str)  
# print(db_conn_str)
a = pd.DataFrame({'a':[1,2,3,],'b':['dddddd','3','5']})

a.to_sql(name='test_blob', con=con, if_exists='append', dtype={'b':BLOB},index=False)

```

#### Problem description
<details>
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\base.py in _execute_context(self, dialect, constructor, statement, parameters, *args)
   1174 
-> 1175             context = constructor(dialect, self, conn, *args)
   1176         except BaseException as e:

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\default.py in _init_compiled(cls, dialect, connection, dbapi_connection, compiled, parameters)
    833                         )
--> 834                         for key in compiled_params
    835                     )

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\default.py in <genexpr>(.0)
    833                         )
--> 834                         for key in compiled_params
    835                     )

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\sql\sqltypes.py in process(value)
    912             if value is not None:
--> 913                 return DBAPIBinary(value)
    914             else:

D:\Anaconda3\envs\py3-back\lib\site-packages\pymysql\__init__.py in Binary(x)
     84     else:
---> 85         return bytes(x)
     86 

TypeError: string argument without an encoding

The above exception was the direct cause of the following exception:

StatementError                            Traceback (most recent call last)
<ipython-input-66-e1bff449d290> in <module>
      6 a = pd.DataFrame({'a':[1,2,3,],'b':['dddddd','3','5']})
      7 
----> 8 a.to_sql(name='test_blob', con=con, if_exists='append', dtype={'b':BLOB},index=False)

~\AppData\Roaming\Python\Python36\site-packages\pandas\core\generic.py in to_sql(self, name, con, schema, if_exists, index, index_label, chunksize, dtype, method)
   2710             chunksize=chunksize,
   2711             dtype=dtype,
-> 2712             method=method,
   2713         )
   2714 

~\AppData\Roaming\Python\Python36\site-packages\pandas\io\sql.py in to_sql(frame, name, con, schema, if_exists, index, index_label, chunksize, dtype, method)
    516         chunksize=chunksize,
    517         dtype=dtype,
--> 518         method=method,
    519     )
    520 

~\AppData\Roaming\Python\Python36\site-packages\pandas\io\sql.py in to_sql(self, frame, name, if_exists, index, index_label, schema, chunksize, dtype, method)
   1318         )
   1319         table.create()
-> 1320         table.insert(chunksize, method=method)
   1321         if not name.isdigit() and not name.islower():
   1322             # check for potentially case sensitivity issues (GH7815)

~\AppData\Roaming\Python\Python36\site-packages\pandas\io\sql.py in insert(self, chunksize, method)
    754 
    755                 chunk_iter = zip(*[arr[start_i:end_i] for arr in data_list])
--> 756                 exec_insert(conn, keys, chunk_iter)
    757 
    758     def _query_iterator(

~\AppData\Roaming\Python\Python36\site-packages\pandas\io\sql.py in _execute_insert(self, conn, keys, data_iter)
    668         """
    669         data = [dict(zip(keys, row)) for row in data_iter]
--> 670         conn.execute(self.table.insert(), data)
    671 
    672     def _execute_insert_multi(self, conn, keys, data_iter):

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\base.py in execute(self, object_, *multiparams, **params)
    982             )
    983         else:
--> 984             return meth(self, multiparams, params)
    985 
    986     def _execute_function(self, func, multiparams, params):

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\sql\elements.py in _execute_on_connection(self, connection, multiparams, params)
    291     def _execute_on_connection(self, connection, multiparams, params):
    292         if self.supports_execution:
--> 293             return connection._execute_clauseelement(self, multiparams, params)
    294         else:
    295             raise exc.ObjectNotExecutableError(self)

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\base.py in _execute_clauseelement(self, elem, multiparams, params)
   1101             distilled_params,
   1102             compiled_sql,
-> 1103             distilled_params,
   1104         )
   1105         if self._has_events or self.engine._has_events:

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\base.py in _execute_context(self, dialect, constructor, statement, parameters, *args)
   1176         except BaseException as e:
   1177             self._handle_dbapi_exception(
-> 1178                 e, util.text_type(statement), parameters, None, None
   1179             )
   1180 

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\base.py in _handle_dbapi_exception(self, e, statement, parameters, cursor, context)
   1480             elif should_wrap:
   1481                 util.raise_(
-> 1482                     sqlalchemy_exception, with_traceback=exc_info[2], from_=e
   1483                 )
   1484             else:

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\util\compat.py in raise_(***failed resolving arguments***)
    176 
    177         try:
--> 178             raise exception
    179         finally:
    180             # credit to

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\base.py in _execute_context(self, dialect, constructor, statement, parameters, *args)
   1173                 conn = self._revalidate_connection()
   1174 
-> 1175             context = constructor(dialect, self, conn, *args)
   1176         except BaseException as e:
   1177             self._handle_dbapi_exception(

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\default.py in _init_compiled(cls, dialect, connection, dbapi_connection, compiled, parameters)
    832                             else compiled_params[key],
    833                         )
--> 834                         for key in compiled_params
    835                     )
    836 

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\engine\default.py in <genexpr>(.0)
    832                             else compiled_params[key],
    833                         )
--> 834                         for key in compiled_params
    835                     )
    836 

D:\Anaconda3\envs\py3-back\lib\site-packages\sqlalchemy\sql\sqltypes.py in process(value)
    911         def process(value):
    912             if value is not None:
--> 913                 return DBAPIBinary(value)
    914             else:
    915                 return None

D:\Anaconda3\envs\py3-back\lib\site-packages\pymysql\__init__.py in Binary(x)
     83         return bytearray(x)
     84     else:
---> 85         return bytes(x)
     86 
     87 

StatementError: (builtins.TypeError) string argument without an encoding
[SQL: INSERT INTO test_blob (a, b) VALUES (%(a)s, %(b)s)]
[parameters: [{'a': 1, 'b': 'dddddd'}, {'a': 2, 'b': '3'}, {'a': 3, 'b': '5'}]]

</details>
