As rickycook mentions in another comment, pip-tools is a great solution for maintaining the distinction between allowed version ranges and locked, fully qualified versions for the given environment.

I've been using pip-tools with tox for a couple of years now. I maintain a requirements.in and requirements.testing.in, and then I can run

  $ tox -e pip-compile

to generate my fully qualified requirements. The pip-compile command is handled by a tox.ini section.

  [testenv:pip-compile]
  commands =
      pip-compile {posargs}
      pip-compile -o requirements.testing.txt {posargs} requirements.testing.in
  deps =
      pip
      pip-tools

The remaining nasty part is automated extraction requirements for setup.py's install_requires and dependency_links. I wrote a function to handle VCS links and other complicated syntax that I'm copying around to all of my projects. Otherwise, pip-tools has been a great solution.