Consider adding validation when Python interpreter constraints conflict with platforms

[Eric-Arellano]

Soon, we will fix platforms in V2 to override whatever the --interpreter-constraint flag would have been that we normally pass to Pex. As explained in pex-tool/pex#957, it does not make sense to specify both flags.

But, this means that it is possible, for example, for the transitive closure of a python_binary to have Python 2.7 interpreter constraints, but a platform of linux-x86_64-cp-37-cp37m. We would end up building a Pex compatible with Py37, even though the code would normally use Py27. Arguably, it would be better to warn/error with a nice error message here.

Implementation wise, we could leverage pkg_resources:

from pkg_resources import Requirement
>>> r = Requirement.parse("CPython>=3.4,!=3.6.4,!=3.7.0")
>>> "3.6" in r
True
>>> "3.6.4" in r
False
>>> "3.7.0" in r
False
>>> "3.7" in r
False
>>> "3.7.1" in r
True