diff --git a/contextlib2/__init__.pyi b/contextlib2/__init__.pyi
index 5fe29b7..d42c6b0 100644
--- a/contextlib2/__init__.pyi
+++ b/contextlib2/__init__.pyi
@@ -19,8 +19,11 @@ from typing import (
 )
 from typing_extensions import ParamSpec, Protocol
 
-# Note: the various 'if True:' guards replace sys.version checks in the
-# original typeshed file that don't apply to the contextlib2 backport API
+# contextlib2 API adaptation notes:
+# * the various 'if True:' guards replace sys.version checks in the original
+#   typeshed file (those APIs are available on all supported versions)
+# * deliberately omitted APIs are listed in `dev/mypy.allowlist`
+#   (e.g. deprecated experimental APIs that never graduated to the stdlib)
 
 AbstractContextManager = ContextManager
 if True:
@@ -119,7 +122,11 @@ if True:
         ) -> Awaitable[bool]: ...
 
 if True:
-    @overload
-    def nullcontext(enter_result: _T) -> ContextManager[_T]: ...
-    @overload
-    def nullcontext() -> ContextManager[None]: ...
+    class nullcontext(AbstractContextManager[_T]):
+        enter_result: _T
+        @overload
+        def __init__(self: nullcontext[None], enter_result: None = ...) -> None: ...
+        @overload
+        def __init__(self: nullcontext[_T], enter_result: _T) -> None: ...
+        def __enter__(self) -> _T: ...
+        def __exit__(self, *exctype: Any) -> bool: ...
diff --git a/dev/mypy.allowlist b/dev/mypy.allowlist
new file mode 100644
index 0000000..e9e26a9
--- /dev/null
+++ b/dev/mypy.allowlist
@@ -0,0 +1,3 @@
+# Deprecated APIs that never graduated to the standard library
+contextlib2.ContextDecorator.refresh_cm
+contextlib2.ContextStack
diff --git a/dev/py3_10_contextlib_pyi_to_contextlib2_pyi.patch b/dev/py3_10_contextlib_pyi_to_contextlib2_pyi.patch
index 6e1f7e1..12a7007 100644
--- a/dev/py3_10_contextlib_pyi_to_contextlib2_pyi.patch
+++ b/dev/py3_10_contextlib_pyi_to_contextlib2_pyi.patch
@@ -1,5 +1,5 @@
---- ../contextlib.pyi	2021-06-26 21:36:16.491964153 +1000
-+++ contextlib2/__init__.pyi	2021-06-26 21:41:08.109598690 +1000
+--- ../contextlib.pyi	2021-06-27 16:02:28.004872421 +1000
++++ contextlib2/__init__.pyi	2021-06-27 16:00:25.431733524 +1000
 @@ -1,3 +1,6 @@
 +# Type hints copied from the typeshed project under the Apache License 2.0
 +# https://github.com/python/typeshed/blob/64c85cdd449ccaff90b546676220c9ecfa6e697f/LICENSE
@@ -7,12 +7,15 @@
  import sys
  from types import TracebackType
  from typing import (
-@@ -16,8 +19,11 @@
+@@ -16,8 +19,14 @@
  )
  from typing_extensions import ParamSpec, Protocol
  
-+# Note: the various 'if True:' guards replace sys.version checks in the
-+# original typeshed file that don't apply to the contextlib2 backport API
++# contextlib2 API adaptation notes:
++# * the various 'if True:' guards replace sys.version checks in the original
++#   typeshed file (those APIs are available on all supported versions)
++# * deliberately omitted APIs are listed in `dev/mypy.allowlist`
++#   (e.g. deprecated experimental APIs that never graduated to the stdlib)
 +
  AbstractContextManager = ContextManager
 -if sys.version_info >= (3, 7):
@@ -20,7 +23,7 @@
      AbstractAsyncContextManager = AsyncContextManager
  
  _T = TypeVar("_T")
-@@ -35,7 +41,7 @@
+@@ -35,7 +44,7 @@
  # type ignore to deal with incomplete ParamSpec support in mypy
  def contextmanager(func: Callable[_P, Iterator[_T]]) -> Callable[_P, _GeneratorContextManager[_T]]: ...  # type: ignore
  
@@ -29,7 +32,7 @@
      def asynccontextmanager(func: Callable[_P, AsyncIterator[_T]]) -> Callable[_P, AsyncContextManager[_T]]: ...  # type: ignore
  
  class _SupportsClose(Protocol):
-@@ -46,7 +52,7 @@
+@@ -46,7 +55,7 @@
  class closing(ContextManager[_SupportsCloseT]):
      def __init__(self, thing: _SupportsCloseT) -> None: ...
  
@@ -38,7 +41,7 @@
      class _SupportsAclose(Protocol):
          async def aclose(self) -> object: ...
      _SupportsAcloseT = TypeVar("_SupportsAcloseT", bound=_SupportsAclose)
-@@ -88,7 +94,7 @@
+@@ -88,7 +97,7 @@
          __traceback: Optional[TracebackType],
      ) -> bool: ...
  
@@ -47,12 +50,12 @@
      _S = TypeVar("_S", bound=AsyncExitStack)
  
      _ExitCoroFunc = Callable[[Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]], Awaitable[bool]]
-@@ -112,7 +118,7 @@
+@@ -112,7 +121,7 @@
              __traceback: Optional[TracebackType],
          ) -> Awaitable[bool]: ...
  
 -if sys.version_info >= (3, 7):
 +if True:
-     @overload
-     def nullcontext(enter_result: _T) -> ContextManager[_T]: ...
-     @overload
+     class nullcontext(AbstractContextManager[_T]):
+         enter_result: _T
+         @overload
diff --git a/tox.ini b/tox.ini
index 5f7972a..43a90fa 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,7 +8,7 @@ commands =
     coverage report
     coverage xml
     # mypy won't install on PyPy, so only run the typechecking on CPython
-    !pypy3: mypy contextlib2
+    !pypy3: python -m mypy.stubtest --allowlist dev/mypy.allowlist contextlib2
 deps =
     coverage
     !pypy3: mypy