Support type hints and mypy

[altendky]

I am just starting into this but figured I'd get a ticket in sooner than later. Hopefully supporting type hints requires pretty minimal effort in qtpy since it is just (primarily?) a pass through library. I have had at least preliminary success by adding a py.typed file and then using mypy's --always-true and --always-false to set PYSIDE2 and PYQT5 variables as they need to be for the library I am interested in. I imagine giving qtpy a cli such that you could call env/bin/mypy --target yourlib $(env/bin/qtpy mypyargs) and the qtpy cli would generate the proper mypy arguments based on the regular library selection code. For the sake of allowing mypy runs to really be static I would expect to also document everything to set and allow generation of args via the cli based on a forced library like env/bin/qtpy mypyargs --use pyside2 or such.

Opinions? What sort of target would I have to hit to have a PR for this ready for the next release?

[altendky]

Or maybe a mypy plugin could do this and would make more sense? I'll have to look into that.