…t present.

This clarifies that it is a deliberate choice instead of implementing the “correct” semantics.

The next commit will add a lot of test cases that will use this.

Fixes psf#3581.

The old algorithm checked, for each target version 3.X, whether 3.X (which is roughly the same as 3.X.0) is contained in a version specifier that was modified from the requires-python project metadata to be less strict regarding patch versions. One problem of it was that, for a requires-python value of ">3.X" (which means the same as ">3.X.0"), it concluded that target version 3.X is not supported although Python versions >= 3.X.1 are included in the version specifier. I found the old approach hard to reason about and hard to fix.

To correctly check whether a target version 3.X is supported, the algorithm must check whether 3.X.* overlaps the version specifier in the requires-python project metadata. Checking only specific versions (like 3.X.0) is not sufficient in general. The `packaging` library, which implements the logic for (PEP440-compatible) versions and version specifiers, doesn’t implement checking for overlap of two version specifiers, however.

The new algorithm works by converting the version specifiers to interval sets, which are then checked for overlap.