add RaisesGroup & Matcher

[jakkdl]

Previous PR's: #11656 (ExpectedExceptionGroup), #11671 (closed to give this a fresh PR w/o stale comments)
fixes: #11538 #12504

This adds RaisesGroup (also exported as raises_group) and Matcher, to allow a robust way of expecting ExceptionGroup and the ability to specify the structure of nested groups, etc etc.

with RaisesGroups(ValueError):
    raise ExceptionGroup("", (ValueError(),))

This is the exact implementation available in trio.testing.RaisesGroup which has been in use since december 2023 and gone through a couple iterations since to settle on a good interface. The most recent addition was python-trio/trio#3145 which added error messages, where the formatting can probably still be fine-tuned.

Pytest currently has excinfo.group_contains for checking exceptiongroups... which is very problematic. I think it should be deprecated, either in the same release as this PR, or very soon after.

This currently does not touch the implementation of pytest.raises, but if we made Matcher a contextmanager we could easily support

with Matcher(ValueError):
    raise ValueError()

this would fix e.g. #12763 with Matcher having the check parameter. The only complication is the legacy form of pytest.raises but that should be solvable with a small helper function.
This can however be done in a followup PR.

This currently does not add an .assert_matches method, which previous iterations of the PR had. The reason to add that would be to improve the error message on failure, though it's possible to get the same result with

assert (m := Matcher(TypeError)).matches(e), m.fail_reason
# versus
Matcher(TypeError).assert_matches(e)

though note that .matches() is already a fairly niche feature that is entirely separate to the context-manager use (not to be confused with the match parameter).

TODO:

• bump the issue # on the newsfragment file
• documentation

[RonnyPfannschmidt]

We ought not to use such a generic name easily

There's a general wishlist plan to enable matchers as a whole that act similar to dirty equals

The matcher api that's being used here conflates multiple aspects of that in a manner that's problematic to partition later

Ideally the whole api around matchers is first evolved as a plugin to avoid early api lock in as it's hard to make good matchers

I'll elaborate further when the flu is over

[jakkdl]

sure, I can rename it... maybe something that's closer to RaisesGroup... RaisesExc/RaisesException? or something like ExpectedExc/ExpectedException

This has been available & in use for >1 year in trio, which has allowed the interface to mature. I don't think pytest needs to repeat that process by putting it in a plugin.

I know you've mentioned matcher objects in #11538 and I'm excited for what that might entail in the future, but I don't think we want to wait for that to add usable exceptiongroup support. If you have concrete ways of improving the RaisesGroup interface to harmonize with it I'm of course open to suggestions though.