Improve missing backend error

[EmmanuelNiyonshuti]

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Changes

Fixes #.

Right now when you have a backend like trio in BACKENDS but it is not installed, the error says:

LookupError: No such backend: trio

This is a bit confusing because trio is a valid backend, it is just not installed. I changed it to tell the user what to do instead:

LookupError: Backend 'trio' is not available. Install it with: pip install anyio[trio]

The fix uses the already existing BACKENDS constant to check if the backend is known but just not installed, and gives a different message for that case.

Checklist

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• You've added tests (in tests/) 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).

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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)

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[agronholm]

Looks good. I just need a number of minor changes first.

[agronholm]

LGTM now. Thanks!