Add python type annotations

[chadrik]

Hi, it would be very valuable if this project introduced type annotations to aid in code completion and static analysis for projects that use this one.

If you're open to accepting a series of PRs for this, I can do a lot of the ground work.

Here's a series of PRs I made to add type hints to Apache Beam: https://github.com/apache/beam/pulls?q=is%3Apr+author%3Achadrik++typing++is%3Amerged

Here's a recent one I have open for Rez: AcademySoftwareFoundation/rez#1761

Here are some links on static typing:

• Python Type Checking (Guide)
• mypy
• pyright (used by VC Code)