Add mypy check in test

[qria]

Add mypy check for type safety.

[qria]

Fun issues encountered with mypy until now:

• Missing function on pkg_resources.EntryPoint python/typeshed#1463

• (no issue yet) Missing stub for pkg_resources.Distribution.PKG_INFO

• Docstring for enum is not inferred as string. python/mypy#5483

• (no issue yet, possibly non-issue) reveal_type(A.doc) is always Optional[str] even if docstring is declared

• Declare and use attributes of function objects python/mypy#2087

We might want to wait implementing mypy until it is sufficiently matured.