Add _typeshed.Incomplete (#7535)

Closes: #5613

Author: srittau

stdlib/_typeshed/__init__.pyi

@@ -8,7 +8,7 @@ import mmap
 import sys
 from os import PathLike
 from typing import AbstractSet, Any, Awaitable, Container, Generic, Iterable, Protocol, TypeVar
-from typing_extensions import Final, Literal, final
+from typing_extensions import Final, Literal, TypeAlias, final
 
 _KT = TypeVar("_KT")
 _KT_co = TypeVar("_KT_co", covariant=True)
@@ -23,6 +23,13 @@ _T_contra = TypeVar("_T_contra", contravariant=True)
 #   def __enter__(self: Self) -> Self: ...
 Self = TypeVar("Self")  # noqa Y001
 
+# For partially known annotations. Usually, fields where type annotations
+# haven't been added are left unannotated, but in some situations this
+# isn't possible or a type is already partially known. In cases like these,
+# use Incomplete instead of Any as a marker. For example, use
+# "Incomplete | None" instead of "Any | None".
+Incomplete: TypeAlias = Any
+
 # stable
 class IdentityFunction(Protocol):
     def __call__(self, __x: _T) -> _T: ...