[3.14] gh-132762: Fix underallocation bug in `dict.fromkeys()`(gh-133627) (gh-133685)

The function `dict_set_fromkeys()` adds elements of a set to an existing
dictionary. The size of the expanded dictionary was estimated with
`PySet_GET_SIZE(iterable)`, which did not take into account the size of the
existing dictionary.
(cherry picked from commit 421ba589d02b53131f793889d221ef3b1f1410a4)

Co-authored-by: Angela Liss <59097311+angela-tarantula@users.noreply.github.com>