Prefer target Python version over current version for builds (#1040)
Extends #1029
Closes https://github.com/astral-sh/puffin/issues/1038
Instead of always using the current Python version for builds when a
target version is provided, we will do our best to use a compatible
Python version for builds.
Removes behavior where Python versions without patch versions were
always assumed to be the latest known patch version (previously
discussed in https://github.com/astral-sh/puffin/pull/534). While this
was convenient for resolutions which include packages which require
minimum patch versions e.g. `requires-python=">=3.7.4"`, it conflicts
with the idea that the target Python version you provide is the
_minimum_ compatible version. Additionally, it complicates interpreter
lookup as we cannot tell if the user has asked for that specific patch
version or not.
