Switch backend detection to use entrypoints.

[Fuyukai]

Changes

This allows for external libraries to provide their own custom async backends, without needing to hack anyio internals.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

* Fix big bad boo-boo in task groups (#123 <https://github.com/agronholm/anyio/issues/123>_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.