[CI] Tests only run on Python 3.10 while the package supports 3.10+

[V2arK]

setup.py declares python_requires=">=3.10" and the wheel is py3-none-any,
so the supported range is 3.10 through whatever is current. CI only ever runs
3.10.

Workflow	Python
unit_tests.yml	3.10
lint.yml	3.10
publish.yml	3.11

Support for 3.12 was widened deliberately: #87 was a user reporting they could
not install the package, and #88 (2025-03-18) changed python_requires from
"<3.12" to ">=3.10". That PR touched setup.py only — CI was never
extended to cover the versions we had just started supporting.

The concrete miss

centml/sdk/utils/client_certs.py called datetime.utcnow() from June 2024
until #154 landed this week. CPython started emitting a DeprecationWarning
for it in 3.12, so:

• a developer running 3.12 locally saw four warnings per certificate generation
• CI on 3.10 saw none, every run, for fifteen months

A user on 3.12 with -W error or pytest's filterwarnings = error would have
hit a hard failure that CI could not have caught.

Suggestion

Give the test workflow a matrix so the declared floor and the versions users
actually run are both exercised:

strategy:
  matrix:
    python-version: ["3.10", "3.12"]
steps:
  - uses: actions/setup-python@v3
    with:
      python-version: ${{ matrix.python-version }}

3.13 is worth considering too. Lint and typecheck are lower value to matrix —
they mostly re-check the same source — but the test job is where a
version-specific behaviour difference would surface.

Worth noting separately: publish.yml builds on 3.11, a version no job ever
tests on. Harmless for a pure-Python py3-none-any wheel, but it means the
published artefact is produced by an interpreter that never ran the suite.