python.el: Indent docstring lines to base-indent

python.el: Indent docstring lines to base-indent

Fixes: debbugs:19595
Thanks to immerrr <immerrr@gmail.com> for reporting and providing
an initial patch.

* lisp/progmodes/python.el
(python-indent-context): Add :inside-docstring context.
(python-indent--calculate-indentation): Handle :inside-docstring.
(python-indent-region): Re-indent docstrings.

* test/automated/python-tests.el (python-indent-region-5)
(python-indent-inside-string-2): Fix tests.