fix: the Python matrix and classifiers cover the whole supported range

[AlexeyShalaev]

The generated unit-test matrix was ["{{ python_min_version }}", "3.12", "3.13"]. With 3.12 as the minimum it tested 3.12 twice. With 3.14 it tested 3.12 and 3.13, below the project's own requires-python, and with any minimum nothing ran on 3.14, which every library in the org tests. The classifiers listed only the minimum, and the Codecov upload was tied to 3.12, so a render with a higher minimum uploaded nothing.

Both lists are now generated from python_min_version to python_max_version. The latter is a hidden value in copier.yml (when: false, so it is not asked and not written to the answers file), set to 3.14: the next Python is one line. Coverage is uploaded from the newest version. python_min_version gets a validator, a 3.x from 3.10 to the max; 3.10 is the floor because redis-client-kit uses it.

I kept the max at 3.14, not 3.15: pg-partsmith#121 shows that a dependency set still needs bumps (greenlet, pywin32) before 3.15 installs everywhere, and a fresh render resolves whatever is newest.

Renders of this branch:

min 3.10  matrix ["3.10", "3.11", "3.12", "3.13", "3.14"]  classifiers 3.10 … 3.14  coverage on 3.14
min 3.12  matrix ["3.12", "3.13", "3.14"]                  classifiers 3.12 3.13 3.14
min 3.14  matrix ["3.14"]                                  classifiers 3.14
3.9, 3.15, 4.1, 3.x   Validation error for question 'python_min_version': Use a 3.x version from 3.10 to 3.14

The answers file of a render has no python_max_version. actionlint 1.7.12 on the rendered workflows: only the two known client-id findings in release-please.yml.

Found while porting the template fixes into the libraries.