Do not import pep8 always/unnecessarily

[sigmavirus24]

In GitLab by @blueyed on Jan 8, 2019, 14:07

In https://gitlab.com/pycqa/flake8-polyfill/blob/8664f8e3186ecb0dfdfa057787dc9a6f426ed32c/src/flake8_polyfill/stdin.py#L2-5 pep8 gets imported always, which might trigger https://gitlab.com/pycqa/flake8-docstrings/issues/28 (because pycodestyle's wheel itself ships pep8.py (PyCQA/pycodestyle#745)).

I think it makes sense to only try to import it on demand in general, i.e. it would not import it when only pycodestyle should get patched.