Patch python_full_version unconditionally

[jaraco]

Closes #678 . Reprise of #802.

• Extract a method for repairing the python_full_version.
• Repair the version even if passed in.
• Update test to reflect the new expectation.

[sbidoul]

I don't have a strong opinion on this. In #802 I did the smallest possible change to unblock pip.

In the end the root cause applies to all environment markers (#774).

[jaraco]

In the end the root cause applies to all environment markers (#774).

Indeed, this change won't affect those failures, because they're related to platform_release. At least with python_full_version, there's a pretty constrained space of potential versions (whatever CPython or its variants produce).

I'm somewhat disinterested in solving the larger issue, given that I was happy with distutils' lenient version objects.