[3.14] gh-145754: Update signature retrieval in unittest.mock to use forwardref annotation format (GH-145756) (#146191)

gh-145754: Update signature retrieval in unittest.mock to use forwardref annotation format (GH-145756)
(cherry picked from commit d357a7dbf38868844415ec1d5df80379ea5a2326)

Co-authored-by: Matthias Schoettle <git@mattsch.com>