[mailbox] Remove _ProxyFile.__class_getitem__ in Python 3.15 (#16322)

Source: python/cpython#140838

Author: donbarbos

stdlib/@tests/stubtest_allowlists/py315.txt

@@ -8,7 +8,6 @@ _frozen_importlib_external.SourceLoader.source_to_code
 dataclasses.MISSING
 dataclasses._MISSING_TYPE
 dataclasses.field
-mailbox._ProxyFile.__class_getitem__
 
 # =============================================================
 # Allowlist entries that cannot or should not be fixed; >= 3.15

stdlib/mailbox.pyi

@@ -294,7 +294,8 @@ class _ProxyFile:
     def flush(self) -> None: ...
     @property
     def closed(self) -> bool: ...
-    def __class_getitem__(cls, item: Any, /) -> GenericAlias: ...
+    if sys.version_info < (3, 15):
+        def __class_getitem__(cls, item: Any, /) -> GenericAlias: ...
 
 class _PartialFile(_ProxyFile):
     def __init__(self, f: _GetFileReturn, start: int | None = None, stop: int | None = None) -> None: ...