Add support for pre-release and development versions

[jwhitlock]

PyPi has the concept of pre-release and development versions, which could be installed with:

pip install --pre django-nose

Published pre-release versions could include PRs that "work for me", and get wider testing, such as optional builds in continuous integration environments and manual installs in personal projects. This could help move several PRs that are stuck because they do not have integration tests that demonstrate the issue or that the change fixes them.

It is possible that setuptools_scm has features that would make pre-release versions easy for maintainers.