Add small Python 3.15 stdlib APIs

[JelleZijlstra]

Summary

• Add small Python 3.15 APIs in pyexpat, stat, symtable, unicodedata, and zlib.
• Remove corresponding 3.15 stubtest allowlist entries.

Relevant report entries

• Added pyexpat.XMLParserType.SetBillionLaughsAttackProtectionActivationThreshold() and SetBillionLaughsAttackProtectionMaximumAmplification() (CPython source: https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Modules/pyexpat.c)
• Added stat.STATX_ATTR_* constants for Linux statx attributes (CPython source: https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Lib/stat.py)
• Added symtable.Function.get_cells and symtable.Symbol.is_cell (CPython PR: expose CELL status of a symbol in symtable cpython#143504; source: https://github.com/python/cpython/blob/main/Lib/symtable.py)
• Added unicodedata.block, unicodedata.extended_pictographic, unicodedata.grapheme_cluster_break, unicodedata.indic_conjunct_break, unicodedata.isxidstart, unicodedata.isxidcontinue, and unicodedata.iter_graphemes(start=..., end=...) (CPython PRs: unicodedata module needs way of accurately determining XID_START and XID_CONTINUE properties. cpython#129117, Add unicode grapheme cluster break algorithm cpython#74902, Add block info to unicodedata cpython#66802; source: https://github.com/python/cpython/blob/main/Modules/unicodedata.c)
• Added zlib.adler32_combine and zlib.crc32_combine (CPython PR: Export zlib.{adler,crc}32_combine cpython#134635; source: https://github.com/python/cpython/blob/main/Modules/zlibmodule.c)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
- pandas/core/computation/ops.py:328: error: Need type annotation for "_binary_ops_dict" (hint: "_binary_ops_dict: dict[<type>, <type>] = ...")  [var-annotated]