Fix issue #1433: parse requirements in markers

* InstallRequirement supports PEP 426 markers
* RequirementSet.add_requirement() ignores an InstallRequirement if
  markers don't match.

fixes #1433
> I guess "version 6.0" means "setuptools 6.0", right? :)

I searched for the version of pip and I found __version__ = "6.0.dev1" in pip/__init__.py. It is not the version of pip?