Python specifiers

[matteius]

Fixes #5195

The issue

Right now it is not possible to specify python specifiers -- only the callout of specific versions.

This allows for specifying python such as:

[requires]
python_version = ">3.8"

[requires]
python_version = ">3.8,<3.11"

I'll be honest and say I am not sure what the side effects of people using different python versions to lock would be, but I am willing to find out.

Another thing, if you say

[requires]
python_version = ">3.8,<=3.10"

then the specifier is not going to install 3.10.4 for example, because that is newer than 3.10. I think that is technically correct as far as specifiers go so <3.11 is better if you intend to allow 3.10 than to say <=3.10, but maybe not what people would expect.