Python: fix escapes in quoted-comments

Summary:
BUG: 386685

Escape characters are not detected in triple-quote comments.
Comments in quotes are literal strings that do not have assignment or operations, so the interpreter ignores them. These are also used to associate documentation with objects, using the `__doc__` attribute (Docstrings [1][2]).

Comments in quotes support the same escapes as normal string. Also, since these can be used as documentation strings, I believe that the correct way is to use the same string escape highlight.

* [1] Python. PEP 257 -- Docstring Conventions: https://www.python.org/dev/peps/pep-0257/
* [2] Python docstrings: http://www.pythonforbeginners.com/basics/python-docstrings

Reviewers: #framework_syntax_highlighting, #kate, dhaumann, cullmann

Reviewed By: #framework_syntax_highlighting, #kate, dhaumann

Subscribers: ngraham, kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D14062