patch 9.1.1472: if_python: PySequence_Fast_{GET_SIZE,GET_ITEM} removed

patch 9.1.1472: if_python: PySequence_Fast_{GET_SIZE,GET_ITEM} removed

Problem:  if_python: PySequence_Fast_{GET_SIZE,GET_ITEM} removed with
          python3.14 in stable ABI
Solution: use PySequence_{Size,GetItem} instead (Zdenek Dohnal)

Python 3.14 removed those two functions from stable API because of
reasoning these function shouldn't be part of stable API at the first
place.

Moving to PySequence_GetSize and PySequence_GetItem fixes the build
failure when Vim is built with dynamic Python and stable API for Python
3.8.

closes: #17575

Signed-off-by: Zdenek Dohnal <zdohnal@redhat.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>