Improve fontification of Python assignments with type hints

Improve fontification of Python assignments with type hints

* lisp/progmodes/python.el (python-font-lock-keywords-maximum-decoration):
Fontify type hints of assignment statement.  (Bug#69357)
* test/lisp/progmodes/python-tests.el
(python-font-lock-assignment-statement-11)
(python-font-lock-assignment-statement-12)
(python-font-lock-assignment-statement-13)
(python-font-lock-assignment-statement-18): Add fontification of type
hints.
(python-font-lock-assignment-statement-19): New test.

(cherry picked from commit c97e7a2da2e5707fc94c2c5e2ddd5f2395cdb80b)