CodingGuidelines: add Python coding guidelines

These are kept short by simply deferring to PEP-8. Most of the Python code in Git is already very close to this style (some things in contrib/ are not). Rationale for version suggestions: - Amongst the noise in [1], there isn't any disagreement about using 2.6 as a base (see also [2]), although Brandon Casey recently added support for 2.4 and 2.5 to git-p4 [3]. - Restricting ourselves to 2.6+ makes aiming for Python 3 compatibility significantly easier [4]. - Advocating Python 3 support in all scripts is currently unrealistic because: - 'p4 -G' provides output in a format that is very hard to use with Python 3 (and its documentation claims Python 3 is unsupported). - Mercurial does not support Python 3. - Bazaar does not support Python 3. - But we should try to make new scripts compatible with Python 3 because all new Python development is happening on version 3 and the Python community will eventually stop supporting Python 2 [5]. - Python 3.1 is required to support the 'surrogateescape' error handler for encoding/decodng filenames to/from Unicode strings and Python 3.0 is not longer supported. [1] http://thread.gmane.org/gmane.comp.version-control.git/210329 [2] http://article.gmane.org/gmane.comp.version-control.git/210429 [3] http://thread.gmane.org/gmane.comp.version-control.git/214579 [4] http://docs.python.org/3.3/howto/pyporting.html#try-to-support-python-2-6-and-newer-only [5] http://www.python.org/dev/peps/pep-0404/ Signed-off-by: Junio C Hamano <gitster@pobox.com>