[tests] support trio's new exception groups, bump min trio rev to 0.25.0

[axiomiety]

Fixes #146.

Technically it's only the tests that are impacted but for completeness I also bumped the trio version on docs-requirements.txt.

[oremanj]

This isn't a user-visible change (it only affected the tests), so I think it should be a misc ("Miscellaneous internal changes") type newsfragment.

The thing about RaisesGroup is not really relevant to what you fixed (you used it in the implementation but it's not the basis of what broke), and mentioning trio 0.24.0 is confusing when this is a change prompted by 0.25.0. I think this can be just "Updated test suite to cope with Trio 0.25.0 and later defaulting strict_exception_groups to True. Trio 0.25.0 is now required to run the tests, although trio-asyncio itself still supports older versions."

[oremanj]

Note the failing docs build here - single backticks in ReST are a cross-reference, and there's no entity named "trio-asyncio". (Also, there's no need for the tilde; the purpose of that is to not print the portion of the xref before the final dot, but there are no dots here.) You can reference the module "trio_asyncio", or double the backticks for fixed-width font without a cross-reference, but I would just drop the backticks and let trio-asyncio be like a normal word; that's the style we try to use in the rest of the documentation.

[oremanj]

Thank you!