CI: Add static analysis / linting to tox

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 #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 https://github.com/python/black/issues/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 / repESP