Typing next steps

[Yobmod]

• Drop 3.6 & add 3.10
• Final 3.6 release
• Add CI tests for early patch versions (e.g. py3.7.1)
• Add __future__.annotations imports
• Replace Optional and Union with | (pep604)
• Re-add TypeGuards
• Replace more casts with TypeGuards
• Check runtime types of TBDs (BearType, TypeGuard, StrongTyping)
• Add more TypedDicts and/or Dataclasses
• Fix no-implicit reexport errors
• Fix other --strict errors

[Byron]

Thanks for all your great work and taking typing even further! Please feel free to ping me whenever you need a release, and if something is particularly important you are welcome to add that to changes.rst as well. I might miss a lot and definitely won't be reproducing finer details due to a lack of awareness.