replace `trio.testing.RaisesGroup` with `pytest.RaisesGroup`

[jakkdl]

pytest 8.4 released RaisesGroup, and it's already better than trio.testing.RaisesGroup with more informative error messages.
I think we can get away with simply aliasing trio.testing.RaisesGroup to pytest.RaisesGroup and trio.testing.Matcher to pytest.RaisesExc, maybe with a short period where we raise a DeprecationWarning if somebody accesses them without pytest available.

We could also start raising warnings straight away for accessing them, given that there's not much reason for keeping them around.

[A5rocks]

IMO it's easiest to mark them as deprecated attributes and not have to worry about interface differences with pytest/aliasing/etc.