Clean up type annotations

[mjpieters]

Typing cleanup

The codebase now passes mypy --strict and pyright type checks, and I've included mypy and pyright in the CI checks to keep it that way.

• Compatibility with older Python versions

  • use Optional and Union instead of ... | None and a | b
  • use typing_extensions.Protocol instead of typing.Protocol
  • use typing.Dict, typing.List, etc. instead of the concrete types.

• Fix incorrect type hints

  • Fix backend .get() annotations; not all were marked as Optional[str]
  • The Coder.decode_as_type() type parameter must be a type to be compatible with ModelField(..., type_=...).
  • Clean up Optional[] use, remove where it is not needed.
  • Clean up variable use in decorator, keeping the raw cached value separate from the return value from the wrapped endpoint.
  • Add targeted ignores and casts where the type checker can't quite see what is going on
  • Annotate the decorator wrapper as returning either the original type or a Response (returning a 304 Not Modified response).
  • Correct type annotation on JsonCoder.decode() to match Coder.decode().

• Code fixes guided by the type checker

  • Don't return anything from Backend.set() methods.
  • Clean up small edge-case in the decorator where response could be None.
  • Correct memcached.get() use of aiomemcache.Client.get(); only pass in the (bytes) key once, and decode the value from bytes to string.

• Use additional typing packages to provide type hints for aiobotocore and redis.

[mjpieters]

Sorry, this PR has ballooned a bit, once I realised there were more issues than just the one little error.

I've included a commit here that expands tests to cover all supported Python versions so that this kind of issue can be caught sooner in future. You may want to update your branch protection rules to require the new test run names.

[long2ice]

Thanks! Please resolve conflicts.

[long2ice]

Thanks!