Enhance Python font-lock to support multilines

Enhance Python font-lock to support multilines

* test/lisp/progmodes/python-tests.el
(python-tests-assert-faces-after-change): New helper function.
(python-font-lock-keywords-level-1-3)
(python-font-lock-assignment-statement-multiline-*): New tests.

* lisp/progmodes/python.el (python-rx): Add `sp-nl' to represent
space or newline (with/without backslash).
(python-font-lock-keywords-level-1)
(python-font-lock-keywords-maximum-decoration): Allow newlines
where appropriate.
(python-font-lock-extend-region): New function.
(python-mode): Set `python-font-lock-extend-region' to
`font-lock-extend-after-change-region-function'.