use `expand=True` for `pformat(...)` in python 3.15+

What's the problem this feature will solve?

pformat is used in a few places throughout pytest (though apparently nothing actually tests these outputs because my attempt at a branch doesn't break any tests!) -- python3.15 adds expand: bool as an option which produces "better" reprs for nested objects:

import pprint

obj = {
    'a' * 12: 1, 'b' * 20: 2,
    'c' * 30: {'d' * 5: 3, 'e' * 20: 3, 'f' * 10: 5, 'g' * 20: 6}, 
}

pprint.pprint(obj)
print('===')
pprint.pprint(obj, expand=True)

{'aaaaaaaaaaaa': 1,
 'bbbbbbbbbbbbbbbbbbbb': 2,
 'cccccccccccccccccccccccccccccc': {'ddddd': 3,
                                    'eeeeeeeeeeeeeeeeeeee': 3,
                                    'ffffffffff': 5,
                                    'gggggggggggggggggggg': 6}}
===
{
 'aaaaaaaaaaaa': 1,
 'bbbbbbbbbbbbbbbbbbbb': 2,
 'cccccccccccccccccccccccccccccc': {
  'ddddd': 3,
  'eeeeeeeeeeeeeeeeeeee': 3,
  'ffffffffff': 5,
  'gggggggggggggggggggg': 6,
 },
}

(this is, admittedly a cherry-picked example -- the expand option seems a lot more likely to go past the requested width option -- not really sure why -- for instance try comparing this object: {'a' * 12: 1, 'b' * 20: 2, 'c' * 30: {'d' * 5: 3, 'e' * 40: 3}})

Describe the solution you'd like

can probably run with something similar to this if it's a good idea -- but needs tests:

$ git diff -- src/_pytest/compat.py
diff --git a/src/_pytest/compat.py b/src/_pytest/compat.py
index d3b2a4696..47bdc4bd0 100644
--- a/src/_pytest/compat.py
+++ b/src/_pytest/compat.py
@@ -327,3 +327,13 @@ else:
                 return func
 
             return decorator
+
+
+if sys.version_info >= (3, 15):
+    import pprint
+
+    pformat = functools.partial(pprint.pformat, expand=True)
+else:
+    import pprint
+
+    pformat = pprint.pformat

Alternative Solutions

shrugs

Additional context

there's definitely a duplicate of this but I could not find it :(

Start in src/_pytest/compat.py and search pytest for its pformat uses. Review the proposed Python-version conditional and existing tests around formatted output, then add coverage for the relevant representations. Done means Python 3.15+ uses pprint's expand option while supported older versions continue to work.