Add stub typing for clients

[jackgene]

Changes

Adds type stubs to classes that clients will interact with, and py.typed so that type checkers know type information is available.

Fixes #980 (kind of)

Per providing type annotations, there are several ways to distribute type information:

• inline type annotations (preferred)
• type stub files included in the package
• a separate companion type stub package
• type stubs in the typeshed repository

This is the second option, and does not touch any of the .py files. If you prefer to go with the first option (preferred, but makes changes to .py files), please have a look at #1074 instead. That PR also includes additional details on how type information was added.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.