python_version environment marker uses wrong precision

[thatch]

PEP 508 says that python_version is {major}.{minor} and that >= is a version-aware-comparison operator.

There are real-world examples like

python_version >= "3.5.2"

that should be interpreted as >= "3.6" but we currently do not. Maybe passing in some kind of precision to VersionIntervals.add_spec would help...

This might also help with supporting "in" for python_version