requirements/Makefile: exclude pkg-resources

Having `pkg-resources` in requirements files is never the right thing as
it is not really a package that one can install. This is basically an
artifact of Debian-based systems providing incorrect metadata to pip.
See [this pip issue](https://github.com/pypa/pip/issues/4022) and
[this Ubuntu issue](https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1635463)
for more background.