Don't register a release if it's not a final release fo Djblets.

It's bad to register a non-final version of Djblets to PyPI, since doing
that will cause people to fetch the new version sooner than we want.
Don't support Python 2.5 or older.

Djblets will no longer install on Python 2.5 or older, even from a
source tarball. It also won't build for release.
Release Djblets 0.8 alpha 1.
Bump to 0.8 alpha 2 dev.
Pull in a newer version of six and use it for metaclasses.

The version of six shipped with Django 1.5 is missing some important
compatibility primitives, like the add_metaclass decorator. I've pulled the
copy shipped with Django 1.6 into our old util/compat directory (which was
empty as of a few days ago) to use until we bump our dependency.

For now, this is used to handle the extension hook metaclasses.

Testing done:
* Ran djblets unit tests
* Ran Review Board unit tests

Reviewed at https://reviews.reviewboard.org/r/4922/