[3.12] gh-127750: Backport some tests for singledispatchmethod (GH-130309) (GH-130340) (GH-130341)

(cherry picked from commit 68c57d6f338ebc16be304e640493e29e1c7c3ef5)

(cherry picked from commit 395335d0ff292a15e26575e06f603304d9161ee1)
(cherry picked from commit 10b32054ad6bce821e3b40101d4414d025be6e36)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>