reqcheck: support PEP-0440 version specifiers

[apevec]

cf. https://www.python.org/dev/peps/pep-0440/#version-specifiers
Hopefully rdopkg could somehow use version handling code in pip, instead of re-writing the parser?

Examples of current parser failures taken from the commit message in https://review.gerrithub.io/249486
python-routes !=2.0,!=2.1,>=1.12.3;python_version=='2.7'
python-routes !=2.0,>=1.12.3; python_version!='2.7'

reqcheck does not parse 'python_version'

python-glance-store !=0.9.0,>=0.7.1 (>=0.9.1 in .spec)

reqcheck does not parse !=

python-sqlalchemy <1.1.0,>=0.9.9 (>=0.9.9 in .spec)

reqcheck does not parse <

[yac]

All these except ; are supposed to be suppored (see VERCMP_TABLE), can you give me an exact reproducer for incorrect behavior?

I'd be shocked if the pip code for this would be reusable but I'm gonna look anyway ;) I remember I considered that in the beginning and decided to do it myself for some reason...