BUG: python engine in pd.read_csv fails to use header=None correctly
- [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.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example
For the following example file:
```
# comment

data_block_1
  300.000000     2.700000     0.070000            1          400     1.04000            2


# comment

data_block_2
000001@0001_sum_particles.mrcs 0001_sum.mrc   587.000000  1268.000000    -51.52270
000001@0001_sum_particles.mrcs 0002_sum.mrc   685.000000  1268.000000    -51.52270
```

```python
df1 = pd.read_csv(fname, skiprows=3, skipfooter=7, delim_whitespace=True, engine='python', comment='#', header=None)
df2 = pd.read_csv(fname, skiprows=3, skipfooter=7, delim_whitespace=True, engine='python', comment='#', header=0)
```


```python
>>> df1
Empty DataFrame
Columns: [0, 1, 2, 3, 4, 5, 6]
Index: []
>>> df2
Empty DataFrame
Columns: [300.000000, 2.700000, 0.070000, 1, 400, 1.04000, 2]
Index: []

```


#### Problem description
Trying to read data_block_1 with the python engine (to be able to use skipfooter) yields an empty DataFrame

##### Expected behaviour 
- When `header=None` is passed to `read_csv()`, the dataframe returned should have shape `(1,7)` and contain the values from `data_block_1` (this does not happen)
- When `header=0` is passed, the values in `data_block_1` end up as `df.columns` of an empty DataFrame (this happens)


#### Expected Output
```
            0         1         2  3    4        5  6
0  300.000000  2.700000  0.070000  1  400  1.04000  2
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-7642-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8
pandas           : 1.0.5
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.2
setuptools       : 49.6.0.post20200814
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
