* lisp/progmodes/python.el (python-nav-end-of-statement): Don't assert
that string-start is strictly greater than last-string-end, because
the string end is a position outside of the string and may therefore
be the same as the following string's start.
* test/lisp/progmodes/python-tests.el (python-nav-end-of-statement-2):
New test.