requirements: Switch to compatible releases

requirements: Switch to compatible releases

In commit ab0c443691, we switched from using commit ranges to fixed
ranges. This was a good idea in so far as it ensures we're providing an
application with dependencies that are guaranteed to work. However,
Patchwork as a project isn't active enough to warrant the continued busy
work effort necessary to keep bumping these versions and it's probably
about time to abandon the experiment. However, rather than switching
back to version ranges, use the compatible releases feature introduced
in PEP 440 [1]. This gives us most of the benefits of ranges but with a
nicer syntax.

[1] https://www.python.org/dev/peps/pep-0440/#compatible-release

Signed-off-by: Stephen Finucane <stephen@that.guru>