STYLE use subprocess to validate flake8 on docstrings instead of Application
According to flake8's creator, [the `Application` class from flake8 isn't meant to be used a library](https://github.com/s-weigand/flake8-nb/issues/111#issuecomment-804997221). As their sole maintainer isn't the original author, the library's internals could well change without warning, and this would break `scripts/validate_docstrings`.

This part probably wants changing:

https://github.com/pandas-dev/pandas/blob/77ea98e9b76f872dd0f2a1f3c8aa7871ae23d8bd/scripts/validate_docstrings.py#L186-L199

We're already writing the docstring to a temporary file, so we could just run `flake8` on that via `subprocess.run`