The files in here are cobbled together to make the _actual_
requirements files, including pins.

To re-pin all requirements, run:

   tox -e pin-requirements

To add a new (direct) requirement that simply didn't exist before, run:

   tox -e pin-requirements -- --no-upgrade

The place to actually add the new requirement is in one of the *.in
files. That is: "base.in" for a general dependency or "test.in" for a
development-only dependency.
