Support Trio via AnyIO

[thearchitector]

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

This ports, as closely as possible, the existing asyncio functionality to AnyIO for the benefit of supporting the Trio runtime.

[thearchitector]

migrating the tests looks like it'll be a fair amount of work. also looks like I won't be able to get away with porting line-for-line in the TokenManager since there are some fairly large changes required there to fit anyio's structured concurrency

[thearchitector]

whether this pattern does what is intended pending the test suite