Bug#1114251: pandas: FTBFS on warnings from pytest 8.4

[Rebecca N. Palmer]

Control: retitle -1 pandas: FTBFS on warnings from pytest 8.4
Control: fixed -1 2.3.1-1

The actual error isn't that, but the 120 subcases of 
pandas/tests/groupby/test_raises.py::test_groupby_raises_category_on_category 
that fail with

E           AssertionError: Caused unexpected warning(s): 
[('PytestWarning', PytestWarning("matching against an empty string will 
*always* pass. If you want to check for an empty message you need to 
pass '^$'. If you don't want to match you should pass `None` or leave 
out the parameter."), 
'/usr/lib/python3/dist-packages/_pytest/raises.py', 624)]

pandas/_testing/_warnings.py:203: AssertionError

As this is a warning about lack of test coverage, not about actual 
results, the current pandas is fine to use.

debci says the trigger is pytest 8.3 -> 8.4.  Given that this appears to 
be a normal (non-urgent) upgrade that is also blocked by several other 
issues, and pandas 2.3 (in experimental) fixes it while causing only a 
few other issues (see #1111280), I intend to work on getting pandas 2.3 
into unstable instead of trying to fix this in 2.2.

If pandas does need to be rebuilt for an urgent reason (e.g. a security 
update), disabling the affected test should work.