ci(py312): add Python 3.12 tox environment for fill tests

[opalo1992]

Add a dedicated py312 tox environment mirroring the existing py3 configuration, to run fill tests explicitly on Python 3.12.

Summary

• Add a dedicated py312 environment to the tox envlist.
• Configure [testenv:py312] to mirror the existing [testenv:py3]
  commands, with basepython = python3.12.

Rationale

• The project already targets Python >=3.11.
• Having an explicit py312 environment ensures that fill tests are
  exercised on the latest stable CPython version and helps catch
  compatibility issues early.

Testing

• tox -e py312 (locally)
• No changes to test logic, only CI configuration.

[SamWilsn]

If we want to do this, we'd probably want to do it with a matrix in our CI. The tox environments between CPython 3.11 and CPython 3.12 would be identical (in contrast to the cpython and pypy environments, which are slightly different).