ENH: Use flake8 to check for PEP8 violations in doctests
- [x] tests added 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Improvement to PR #23381 regarding issue #23154 

Enables `--doctests` flag, resulting in doctests being checked by flake8
Uses [flake8.api.legacy](http://flake8.pycqa.org/en/latest/user/python-api.html) to invoke the tests.

Downsides are:
* > Flake8 3.0.0 presently does not have a public, stable Python API.
* provided options in `flake8.get_style_guide` don't have an effect. Thus `setup.cfg` had to be enhanced.
* returns the count of each error within the complete file, not just the requested function. [Might be confusing]

`python ./scripts/validate_docstrings.py pandas.read_excel`
```
                [...]
                Parameter "convert_float" description should finish with "."
        Errors in doctest sections
                2 F721 syntax error in doctest
                2 F821 undefined name '_make_signature'
```

Help in the form of `Use "flake8 pandas/util/_decorators.py" for further information.` could be added to the output:
```
pandas/util/_decorators.py:320:26: F721 syntax error in doctest
pandas/util/_decorators.py:321:13: F721 syntax error in doctest
pandas/util/_decorators.py:322:15: F821 undefined name '_make_signature'
pandas/util/_decorators.py:322:31: F821 undefined name 'f'
```
