Support Python 3.9

[goodoldneon]

Description

Add Python 3.9 support. This required 2 changes:

• Use typing.Optional and typing.Union instead of the | operator (PEP 604)
• Use typing_extensions.TypeGuard instead of typing.TypeGuard (PEP 647)

Additionally:

• Use 3.9 in CI. Stopped using 3.10 and 3.11 in CI since it's overkill: it seems OK to use only the lowest and highest Python versions we support
• Update minimum typing_extensions to 4.8.0, since this is what fastapi==0.104.0 requires

Issue

61

Review

This PR has a lot of changes but they're very low risk since they're almost entirely type annotations. We have a thorough test suite, so these changes should be OK as long as everything passes

[jpwilliams]

@goodoldneon Can user code can still use the newer features like the | operator even though the types are created using typing.Union?

Also we should update the badge on the README to show 3.9 and maybe supported version ranges of the frameworks too?

[goodoldneon]

Can user code can still use the newer features like the | operator even though the types are created using typing.Union?

Yes, type checkers treat typing.Union the same way as |, so users are free to continue using | in their code.

Also we should update the badge on the README to show 3.9 and maybe supported version ranges of the frameworks too?

The badge will automatically update when we publish next. I added framework versions to the readme