Fix electric indent bug in python-mode after dedenting colon

* list/progmodes/python.el (python-indent-post-self-insert-function):
Use markers instead of positions when reindenting statement(s) after
inserting electric colon to avoid reindenting too many
statements (bug#22663).

* test/lisp/progmodes/python-tests.el (python-indent-electric-colon-2):
Improve test case to also verify the fix of bug#22663.

Copyright-paperwork-exempt: yes

test/lisp/progmodes/python-tests.el

@@ -1161,10 +1161,13 @@ def b()
 if do:
     something()
     else
+outside
 "
    (python-tests-look-at "else")
    (goto-char (line-end-position))
    (python-tests-self-insert ":")
+   (should (= (current-indentation) 0))
+   (python-tests-look-at "outside")
    (should (= (current-indentation) 0))))
 
 (ert-deftest python-indent-electric-colon-3 ()