Fix python_requires to valid PEP 440 syntax

[jborean93]

As reported in python-poetry/poetry#4201 the existing python_requires valid of >=3.6.* is invalid and should be >=3.6. PEP440 only states that the wildcard suffix is used with version matching ==. The ordered sections do not state it allows the same rules as == unlike the section for != does.

This still seems to work fine with pip luckily but tools like poetry seem to be a bit more strict in the intepretation and the upcoming version will error if they encounter this syntax python-poetry/poetry#4201.

[frozencemetery]

I'm not sure I agree/disagree with that interpretation of PEP440 , but it does state that X.Y.0 is equivalent to X.Y, so this change is fine regardless.