Add simple GithubAction for Tox

[ChristianKuehnel]

The low-hanging fruit for automatic checks is probably getting tox executed on Github Actions.

I was able to run flake8 , mypy and docs successfully.

However I failed with the test suite. We probably need to set up some more things in the environment...

The output of the Github Actions is visible here: https://github.com/ChristianKuehnel/llvm-lnt/actions