pd.read_csv does not recognize scientific notation if 'decimal' attribute is set with engine=python
Contents of 'abc.tsv':
```
a	b
1,2	3,5E-1
1,2e2	3,5
```
Code:
```python
>>> import pandas as pd
>>> pd.read_csv('abc.tsv', '\t', decimal=',')
       a     b
0    1.2  0.35
1  120.0  3.50
>>> pd.read_csv('abc.tsv', '\t', decimal=',', engine='python')
       a       b
0    1.2  3,5E-1
1  1,2e2     3.5
```
#### Problem description
The outputs of the two read_csv commands above should be the same. In this case an obvious "solution" is to just use the C-engine, however I have to process a file where I need to use a regular expression as the delimiter.

#### Expected Output
```python
>>> pd.read_csv('abc.tsv', '\t', decimal=',', engine='python')
       a     b
0    1.2  0.35
1  120.0  3.50
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.2.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : None
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0.post20200210
Cython           : 0.29.15
pytest           : 5.3.5
hypothesis       : 5.4.1
sphinx           : 2.4.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.48.0

</details>
