Initial step to have continuous integration

This commit introduces the use of tox, which currently only verifies
that code is formatted according to the black project. More checks will
come in upcoming PRs for codespell, pydocstyle, flake8 and mypy.

A GitHub actions is included that runs tox on new pull requests, so we
get automatic feedback and can block commits that fail any check.
Another (official) action that runs hassfest is included as well, to
make sure we are compatible with Home Assistant.

requirements_test.txt

@@ -0,0 +1,2 @@
+black==20.8b1
+mypy==0.782