[3.13] gh-122981: Fix inspect.getsource() for generated classes with Python base classes (GH-123001) (#123182)

gh-122981: Fix inspect.getsource() for generated classes with Python base classes (GH-123001)

Look up __firstlineno__ only in the class' dict, without searching in
base classes.
(cherry picked from commit f88c14d412522587085ae039ebe70b91d5b4e226)

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