python.el: Enhance docstring detection following PEP-257.

python.el: Enhance docstring detection following PEP-257.

* lisp/progmodes/python.el (python-docstring-at-p): Remove function.
(python-info-assignment-statement-p): New function.
(python-info-assignment-continuation-line-p): Use it.
(python-info-docstring-p): New function.
(python-font-lock-syntactic-face-function)
(python-fill-string): Use it.

* test/automated/python-tests.el (python-info-assignment-statement-p-1)
(python-info-assignment-statement-p-2)
(python-info-assignment-statement-p-3, python-info-docstring-p-1)
(python-info-docstring-p-2, python-info-docstring-p-3)
(python-info-docstring-p-4, python-info-docstring-p-5)
(python-info-docstring-p-6): New tests.