contextlib: Remove explicit base class from ExitStack (#7963)

Fixes #7961

test_cases/stdlib/test_contextlib.py

@@ -0,0 +1,18 @@
+from contextlib import ExitStack
+from typing_extensions import assert_type
+
+
+# See issue #7961
+class Thing(ExitStack):
+    pass
+
+
+stack = ExitStack()
+thing = Thing()
+assert_type(stack.enter_context(Thing()), Thing)
+assert_type(thing.enter_context(ExitStack()), ExitStack)
+
+with stack as cm:
+    assert_type(cm, ExitStack)
+with thing as cm2:
+    assert_type(cm2, Thing)