STYLE: fix line length check of flake8
I think I made a small mistake in https://github.com/pandas-dev/pandas/pull/27076 in adding the E501 code (line length) to the ignore list, as setting the flake8 max line length to 88 should have been sufficient.

Black does not check or correct line length in comments or multiline strings (eg docstrings), so this still need to be catched by flake8