BUG: read_csv fails with float_precision="round_trip" and decimal=","
- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the latest version of pandas.

---

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

```python
import io
import pandas as pd

content = """\
6,5;3,3
3,4;1,0
"""

pd.read_csv(io.StringIO(content), delimiter=";", header=None,
            dtype=float, decimal=",", float_precision="round_trip")

```

#### Problem description

read_csv fails if using `float_precision="round_trip"` and `decimal=","`  (or any other decimal and thousands separator).

It raises an 
```
Traceback (most recent call last):
  File "pandas/_libs/parsers.pyx", line 1152, in pandas._libs.parsers.TextReader._convert_tokens
TypeError: Cannot cast array from dtype('O') to dtype('float64') according to the rule 'safe'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 10, in <module>
  File "/Users/aleserjavec/virtual/py37/lib/python3.7/site-packages/pandas/io/parsers.py", line 676, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/Users/aleserjavec/virtual/py37/lib/python3.7/site-packages/pandas/io/parsers.py", line 454, in _read
    data = parser.read(nrows)
  File "/Users/aleserjavec/virtual/py37/lib/python3.7/site-packages/pandas/io/parsers.py", line 1133, in read
    ret = self._engine.read(nrows)
  File "/Users/aleserjavec/virtual/py37/lib/python3.7/site-packages/pandas/io/parsers.py", line 2037, in read
    data = self._reader.read(nrows)
  File "pandas/_libs/parsers.pyx", line 860, in pandas._libs.parsers.TextReader.read
  File "pandas/_libs/parsers.pyx", line 875, in pandas._libs.parsers.TextReader._read_low_memory
  File "pandas/_libs/parsers.pyx", line 952, in pandas._libs.parsers.TextReader._read_rows
  File "pandas/_libs/parsers.pyx", line 1084, in pandas._libs.parsers.TextReader._convert_column_data
  File "pandas/_libs/parsers.pyx", line 1158, in pandas._libs.parsers.TextReader._convert_tokens
ValueError: could not convert string to float: '6,5'
```

#### Expected Output

```
     0    1
0  6.5  3.3
1  3.4  1.0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.16.2
pytz             : 2018.3
dateutil         : 2.6.1
pip              : 20.1.1
setuptools       : 39.0.1
Cython           : 0.29.13
pytest           : 3.7.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.1.5
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 6.2.1
pandas_datareader: None
bs4              : 4.6.0
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.3.0
numexpr          : 2.6.6
odfpy            : None
openpyxl         : 3.0.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 3.7.1
pyxlsb           : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : 1.1.5
numba            : 0.41.0

</details>
