Implement resolve_python_version()

[t-kalinowski]

Currently, adding a Python constraint like py_require(python_version = ">=3.8") changes the selected Python from 3.11 to 3.13, which then results in errors when trying to install packages that are not compatible yet with the latest Python version (e.g., tensorflow, torch).

This PR implements a custom constraint resolver that prefers returning a bugfix Python release, rather than the bleeding edge.

Some examples:

> resolve_python_version()
[1] "3.11.11"
> resolve_python_version(">=3.9")
[1] "3.11.11"
> resolve_python_version(">=3.9")
[1] "3.11.11"
> resolve_python_version(">=3.11")
[1] "3.11.11"
> resolve_python_version(">=3.12")
[1] "3.12.8"
> resolve_python_version(">=3.13")
[1] "3.13.1"
> resolve_python_version("<=3.13")
[1] "3.11.11"
> resolve_python_version("<=3.11")
[1] "3.11.11"
> resolve_python_version("<3.11")
[1] "3.10.16"
> resolve_python_version("3.14.0a3")
[1] "3.14.0a3"
> resolve_python_version("<=3.12,>=3.9,!=3.11")
[1] "3.10.16"
> resolve_python_version("<=3.12,>=3.9")
[1] "3.11.11"
> resolve_python_version("<=3.12,>=3.9,3.10")
[1] "3.10.16"
> resolve_python_version("3.10,3.11")
Error in `resolve_python_version()`:
! Requested Python version constraints could not be satisfied.
  constraints: "3.10,3.11"
Hint: Call `py_require(<version>, action = "set")` to select a specific Python version.
Run `rlang::last_trace()` to see where the error occurred.
> resolve_python_version(">=3.9,3.12")
[1] "3.12.8"
> resolve_python_version(">=3.9,3.12.1")
[1] "3.12.1"

[edgararuiz]

LGTM!