bug#45679: 28.0.50; python-mode variable font locking is inconsistent

[Doug Davis]

Its seems that the issue pops up when a function signature includes the
trailing return type int:

#+begin_src python
def func(a, b) -> None:
    x = 5
    y = 6
#+end_src

has bad assignment font locking for "x = 5"; while this version:

#+begin_src
def func(a, b):
    x = 5
    y = 6
#+end_src

appears to have expected variable assignment font locking.