gh-140808: Remove __class_getitem__ from mailbox._ProxyFile

Lib/mailbox.py

@@ -2090,8 +2090,6 @@ def closed(self):
             return False
         return self._file.closed
 
-    __class_getitem__ = classmethod(GenericAlias)
-
 
 class _PartialFile(_ProxyFile):
     """A read-only wrapper of part of a file."""

Lib/test/test_genericalias.py

@@ -17,7 +17,7 @@
 from functools import partial, partialmethod, cached_property
 from graphlib import TopologicalSorter
 from logging import LoggerAdapter, StreamHandler
-from mailbox import Mailbox, _PartialFile
+from mailbox import Mailbox
 try:
     import ctypes
 except ImportError:
@@ -117,7 +117,7 @@ class BaseTest(unittest.TestCase):
                      Iterable, Iterator,
                      Reversible,
                      Container, Collection,
-                     Mailbox, _PartialFile,
+                     Mailbox,
                      ContextVar, Token,
                      Field,
                      Set, MutableSet,

Misc/NEWS.d/next/Library/2025-10-31-16-25-13.gh-issue-140808.XBiQ4j.rst

@@ -0,0 +1 @@
+The internal class ``mailbox._ProxyFile`` is no longer a parameterized generic.