tests: depend on flake8 instead of including pep8/pyflakes

g-i includes an old version of pep8 and pyflakes and uses that during "make check".

It (1) doesn't catch all cases newer versions of pycodestyle/pyflakes catch and (2) doesn't test all Python files (3) doesn't work with meson.

Instead of updating just remove them and depend on flake8 instead. To run the cehcks simply run flake8 in the root dir. This also makes it possible to run those checks when using meson and not autotools.

To not get test suite failures on flake8 updates move the checks from "make check" to an extra "make check.quality" target.