BUG: Appending Series with dateutil timezone raises TypeError: unhashable type: 'tzutc'
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
import pandas as pd
from dateutil.parser import parse as dt_parse

date = dt_parse('2018-10-24 07:30:00+00:00')
ser = pd.Series({'date': date})
ser2 = pd.Series({'date': date, 'a': 1.0, 'b': 2.0})
df = pd.DataFrame(columns=['c', 'd'])
# No exception
df2 = df.append(ser, ignore_index=True)
# Exception raised from next line
df2 = df.append(ser2, ignore_index=True)
```
#### Problem description
The last line of code above causes an exception.
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-c167f10681a7> in <module>()
      1 ser2 = pd.Series({'date': date})
----> 2 df2 = df.append(ser, ignore_index=True)

~/vp/venv/lib/python3.5/site-packages/pandas/core/frame.py in append(self, other, ignore_index, verify_integrity)
   5155                               index=index,
   5156                               columns=combined_columns)
-> 5157             other = other._convert(datetime=True, timedelta=True)
   5158             if not self.columns.equals(combined_columns):
   5159                 self = self.reindex(columns=combined_columns)

~/vp/venv/lib/python3.5/site-packages/pandas/core/generic.py in _convert(self, datetime, numeric, timedelta, coerce, copy)
   4064             self._data.convert(datetime=datetime, numeric=numeric,
   4065                                timedelta=timedelta, coerce=coerce,
-> 4066                                copy=copy)).__finalize__(self)
   4067 
   4068     # TODO: Remove in 0.18 or 2017, which ever is sooner

~/vp/venv/lib/python3.5/site-packages/pandas/core/internals.py in convert(self, **kwargs)
   3458 
   3459     def convert(self, **kwargs):
-> 3460         return self.apply('convert', **kwargs)
   3461 
   3462     def replace(self, **kwargs):

~/vp/venv/lib/python3.5/site-packages/pandas/core/internals.py in apply(self, f, axes, filter, do_integrity_check, consolidate, **kwargs)
   3322 
   3323             kwargs['mgr'] = self
-> 3324             applied = getattr(b, f)(**kwargs)
   3325             result_blocks = _extend_blocks(applied, result_blocks)
   3326 

~/vp/venv/lib/python3.5/site-packages/pandas/core/internals.py in convert(self, *args, **kwargs)
   2128 
   2129         if by_item and not self._is_single_block:
-> 2130             blocks = self.split_and_operate(None, f, False)
   2131         else:
   2132             values = f(None, self.values.ravel(), None)

~/vp/venv/lib/python3.5/site-packages/pandas/core/internals.py in split_and_operate(self, mask, f, inplace)
    476             # need a new block
    477             if m.any():
--> 478                 nv = f(m, v, i)
    479             else:
    480                 nv = v if inplace else v.copy()

~/vp/venv/lib/python3.5/site-packages/pandas/core/internals.py in f(m, v, i)
   2118         def f(m, v, i):
   2119             shape = v.shape
-> 2120             values = fn(v.ravel(), **fn_kwargs)
   2121             try:
   2122                 values = values.reshape(shape)

~/vp/venv/lib/python3.5/site-packages/pandas/core/dtypes/cast.py in soft_convert_objects(values, datetime, numeric, timedelta, coerce, copy)
    805     # Soft conversions
    806     if datetime:
--> 807         values = lib.maybe_convert_objects(values, convert_datetime=datetime)
    808 
    809     if timedelta and is_object_dtype(values.dtype):

pandas/_libs/src/inference.pyx in pandas._libs.lib.maybe_convert_objects()

TypeError: unhashable type: 'tzutc'
```
#### Expected Output
No exception. 
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-130-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_HK.UTF-8
LOCALE: en_HK.UTF-8

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 36.2.7
Cython: None
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
