Python 3.15 compatibility fixes

Python 3.15 compatibility fixes

Backport of upstream commit 7bb284ac43 (PR #66473) for Python 3.15 support:
 - Support optional %z and %:z (colon_z) in strptime
 - Explicitly close XML iterparse parser to avoid unclosed iterator ResourceWarning
 - Accept Python 3.15 TypeError / ValueError error message changes in tests
 - Define and export PY315 in pandas.compat

Origin: upstream 7bb284ac438d2b981a7f42e3bdd0fe5ee26aada0
Author: Álvaro Kothe, Maximiliano Curia
Bug: https://github.com/pandas-dev/pandas/issues/66473
Bug-Debian: https://bugs.debian.org/1144864
Forwarded: not-needed

Gbp-Pq: Name python3p15compat.patch