CI: Add static analysis / linting to tox

[jszopi]

Scope:

• pyflakes or pylint. The latter is more thorough but requires accordingly setup to eliminate existing errors in the code, so maybe I'll opt for pyflakes for the short-term and create a follow-up issue.
• pycodestyle (PEP 8). Code style will mostly be taken care of by black (to be set up in Format repo with black and set up pre-commit #45), but black's style is documented to only be a strict subset of PEP 8.
• pydocstyle (PEP 257).
• possibly isort, although black may soon be encroaching onto that territory (see [Feature Request] Optimize imports psf/black#333) and there are already incompatibilities between these two (at time of writing and if I understand correctly)
• consider flake8-bugbear, especially if B006 is not covered by pylint.
• TODO: research enforcing a style within docstrings, which may be possible with pydocstyle or pylint, see SO.

[jszopi]

Changing the title as style will be covered in #45.