[3.13] gh-139905: Provide suggestion in error message if `Generic.__init_subclass__` was not called (GH-139943) (#139956)

gh-139905: Provide suggestion in error message if `Generic.__init_subclass__` was not called (GH-139943)
(cherry picked from commit 5776d0d2e08f4d93dcd62d875bae8c1396a04ba4)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>