Contributing guide / Python coding standards links to Python 2 documents

[Philippe Cloutier]

The Contributing guide's Python coding standards page contains a few links to Python documentation which refer to the Python 2 documentation, in sections Logging and String formatting. For example, "the Python standard library’s logging module" links to version 2.7 of the doc. This is not a bug as these links all still work, but they should rather point to the Python 3 version, for example https://docs.python.org/3/library/logging.html
The new documentation allows linking to a major version, so updating should solve for quite a while.