gh-94439: typing docs: Add minimum version to __required_keys__ and __optional_keys__ (GH-95373)

(cherry picked from commit f235178)

Co-authored-by: Howie Zhao <[email protected]>

Author: miss-islington

Doc/library/typing.rst

@@ -1544,6 +1544,9 @@ These are not used in annotations. They are building blocks for declaring types.
          True
 
    .. attribute:: __required_keys__
+
+      .. versionadded:: 3.9
+
    .. attribute:: __optional_keys__
 
       ``Point2D.__required_keys__`` and ``Point2D.__optional_keys__`` return
@@ -1566,6 +1569,8 @@ These are not used in annotations. They are building blocks for declaring types.
          >>> Point3D.__optional_keys__ == frozenset({'x', 'y'})
          True
 
+      .. versionadded:: 3.9
+
    See :pep:`589` for more examples and detailed rules of using ``TypedDict``.
 
    .. versionadded:: 3.8