Support max version marker of stdlib typeshed modules

[srittau]

This implements python/typeshed#5304.

Test Plan

I verified this manually with the following test script:

import datetime

And running mypy using Python 3.9 in the following situations:

VERSIONS file	mypy parameters	expected result
2.7	none	passes
2.7-3.10	none	passes
2.7-3.9	none	passes
2.7-3.8	none	cannot find stub
2.7	--python-version=3.8	passes
2.7-3.9	--python-version=3.8	passes
2.7-3.8	--python-version=3.8	passes
2.7-3.7	--python-version=3.8	cannot find stub

I did the same with the following test script (as functools has a separate Python 2 stub):

import functools

[srittau]

Cc @JelleZijlstra I have slightly updated the code to support the open-ended 2.7- format.