Use pprint.pformat(expand=True) on Python 3.15+ for better nested reprs

[SakshamChawla76]

Description

Use pprint.pformat(expand=True) on Python 3.15+ to produce better nested reprs for complex data structures.

This addresses issue #14859 which requested using the new expand=True parameter added in Python 3.15.

Changes

• src/_pytest/_io/saferepr.py: Added version check to use expand=True in safeformat() for Python 3.15+

Testing

• All existing saferepr tests pass (pre-existing failures on Python 3.14 are unrelated to this change)
• The new behavior on Python 3.15+ produces better nested reprs:

# Before (and on Python < 3.15):
{'aaaaaaaaaaaa': 1, 'bbbbbbbbbbbbbbbbbbbb': 2, 'cccccccccccccccccccccccccccccc': {'ddddd': 3, 'eeeeeeeeeeeeeeeeeeee': 3}}

# After (on Python 3.15+):
{'aaaaaaaaaaaa': 1,
 'bbbbbbbbbbbbbbbbbbbb': 2,
 'cccccccccccccccccccccccccccccc': {'ddddd': 3,
                                    'eeeeeeeeeeeeeeeeeeee': 3}}