Revert "[3.13] pythongh-126451: Register contextvars.Context to colle…

…ctions.abc.Mapping (pythonGH-126452) (python#126518)"

This reverts commit 59316a6.

Lib/contextvars.py

@@ -1,8 +1,4 @@
-import _collections_abc
 from _contextvars import Context, ContextVar, Token, copy_context
 
 
 __all__ = ('Context', 'ContextVar', 'Token', 'copy_context')
-
-
-_collections_abc.Mapping.register(Context)

Lib/test/test_context.py

@@ -1,4 +1,3 @@
-import collections.abc
 import concurrent.futures
 import contextvars
 import functools
@@ -343,19 +342,6 @@ def ctx2_fun():
 
         ctx1.run(ctx1_fun)
 
-    def test_context_isinstance(self):
-        ctx = contextvars.Context()
-        self.assertIsInstance(ctx, collections.abc.Mapping)
-        self.assertTrue(issubclass(contextvars.Context, collections.abc.Mapping))
-
-        mapping_methods = (
-            '__contains__', '__eq__', '__getitem__', '__iter__', '__len__',
-            '__ne__', 'get', 'items', 'keys', 'values',
-        )
-        for name in mapping_methods:
-            with self.subTest(name=name):
-                self.assertTrue(callable(getattr(ctx, name)))
-
     @isolated_context
     @threading_helper.requires_working_threading()
     def test_context_threads_1(self):