CI: Enforce type checks

[leofang]

We've introduced typing (Python type hints) support on Day 1 of the project, because it helps users understand cuda.core's API design logic. We should ensure our typing is done correctly (as far as mypy etc is concerned).

[leofang]

Nontrivial mypy errors arose from @shwina investigation, from both 3rd and 1st party code. Pushing to beta 4.