[3.13] gh-139076: Fix regression in pydoc not showing extension functions (GH-139077) (GH-139161)

Fix a bug in the pydoc module that was hiding functions in a Python
module if they were implemented in an extension module and the module did
not have __all__.
(cherry picked from commit 7257b24140ac1b39fb8cfd4610134ec79575a396)

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