Update pip download to respect --python-version

[cjerdonek]

Fixes #5369.

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[cjerdonek]

@xavfernandez In this PR, similar to your suggestion here, I'm making e.g. 36 be interpreted as 3.6.0 for the purposes of the "Requires-Python" check. This is the purpose of the unit-tested complete_version_info () function. As you suggested, in a later PR we can start accepting dotted version strings for --python-version (and in particular major-minor-micro, not just major and major-minor).

[xavfernandez]

In some places, we restrict py_version_info to a 3-int-tuple but apparently complete_version_info is capable of letting longer tuple pass unchanged, I think we should be consistent and might want to always have the same type and change the typing hints to Tuple(int, int, int) everywhere...

But after Requires-Python, it is likely that users will also want that pip uses --python-version when evaluating some environment markers like python_version, python_full_version & implementation_version.
And to do so, pip will need more than the 3-int tuple so we'll certainly end up with a 5-tuple as defined by https://docs.python.org/fr/3/library/sys.html#sys.version_info definition but this would certainly be for an other PR :)

[cjerdonek]

@xavfernandez Thanks for the suggestion. I renamed complete_version_info() to normalize_version_info() to reflect that it can now also truncate, and I made it so that the functions used after normalize_version_info() are annotated with Tuple[int, int, int]. There was a bit of an annoyance in that I needed to use typing's cast() to get the fixed-length tuple to pass the mypy check, but it's working now.

By the way, here's an existing issue related to environment markers along the lines you said: #6117 It also has an associated PR (PR #6357) that I gave pointers on.

[cjerdonek]

Thanks again for reviewing, @xavfernandez.

[pradyunsg]

Thanks for doing this @cjerdonek and reviewing this @xavfernandez! :)