Add mypy check to CI workflow.

[toonarmycaptain]

Mypy provides good static type checking. Add it to one or more of the CI tools such that it's reports can be accessed.

Depending on specifics, failing a building a serious issue might be wise.

Related to #120.

[toonarmycaptain]

GitMate.io thinks possibly related issue is #27 (Add installation docs).

[toonarmycaptain]

Expect this to follow on #120, as prematurely adding mypy check will fail.

[toonarmycaptain]

There are some issues where accepting more than one type eg var: Optional[List] or var Union[str, Path, None] results in mypy having issues with the passing of None to things like .joinpath even though such issue would not happen because it is gated by an if var: etc.

[toonarmycaptain]

Forgot to close issue, but mypy has been implemented.