[3.14] gh-132775: Fix _PyFunctIon_VerifyStateless() (gh-134901)

gh-132775: Fix _PyFunctIon_VerifyStateless() ()

The problem we're fixing here is that we were using PyDict_Size() on "defaults",
which it is actually a tuple.  We're also adding some explicit type checks.

This is a follow-up to gh-133221/gh-133528.

(cherry picked from commit dafd14146f7, AKA gh-134900)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>