Python 3.4 support

[Marc Tamlyn]

Python 3.4 support should be added in master ready for 1.7, and probably also in 1.6.X as Py3.4 will be out before Django 1.7. As Py3.4 comes with bundled pip and venv, and is a prime target for new python developers and those running classes, it is very important for Django to support it.

I've run the test suite (on sqlite) against 1.6.X and master. A test run on 1.6.X can be seen at ​https://gist.github.com/mjtamlyn/8195542

The current state of things is as follows:

• There are a couple of warnings printed at startup time. These relate to API changes in plistlib (which is OSX specific) and codecs (universal newlines reading of files has been deprecated).
• Our usage of HTMLParser should now always specify the value of convert_charrefs as it's default will change in Py3.5. This is slightly problematic as Py2.7 does not have this kwarg so we can't universally apply it. Perhaps the best option is a six-like wrapper.
• django.utils.module_loading. module_has_submodule has some issues with eggs. sys.meta_path is giving us importlib as a finder. importlib.find_module is deferred to importlib.find_spec (new in py3.4), which throws an error (ImportError: spec missing loader).
• There is a failing test in the mail library regarding encoding in the mail module. Florian seemed to know about this.
• There is a significant issue with signal deregistration. I've been able to ascertain that something is up in the django.dispatch.saferef module, but I don't have a sufficient understanding to work out what's wrong. There seems to be no test which tests this code directly and failures are thrown up at random during tests, test teardown and/or test suite teardown. They show up either as NoneType is not callable or as catching classes that do not inherit from BaseException is not allowed.