Add support for Trio via AnyIO

[agronholm]

Pull Request check-list

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• Do tests and lints pass with this change?
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• Is the new or changed code fully tested?
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• Is there an example added to the examples folder (if applicable)?
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Description of change

This adds an alternate, AnyIO based asynchronous client. This is necessary in order to make it possible for users of the Trio async framework to use Redis asynchronously. But the potential does not end there. Given that AnyIO supports asyncio as well, it would be possible to later replace the asyncio client with the AnyIO one.

A minor improvement would also be to remove pytest-asyncio from test dependencies and just use the AnyIO test plugin for testing both the AnyIO and asyncio clients.

A couple of things to note:

• Given that redis-entraid is a separate library with asyncio-only async code, I had to limit tests involving that to only run on asyncio. I could also migrate that library as follow-up work.
• This new client must be used with context management due to structured concurrency (task groups being used behind the scenes)
• If the general idea gets a green light, I will make the necessary documentation changes
• Further deduplication of code is possible and I am open to it as follow-up work

[agronholm]

What is the Python version support policy in this project? I see that v6.0.0 was released with Python 3.8 support even though it reached its end of life last year.