python support update --> 3.12

[pp-mo]

[echoix]

Python 3.13 is just 10 days away now

[ocefpaf]

BTW, conda-forge is shipping a py313 version of it and all the tests are passing. You can test it out with:

conda env create --name CFUNITS python=3.13 cf-units pytest -c conda-forge/label/python_rc
conda activate CFUNITS
pytest --pyargs cf_units

[trexfeathers]

We should now be in a good position to add this without missing a spot:

https://github.com/SciTools/cf-units/blob/539dbccd5597b73c1785bbd31d51e4c304e7ffc1/cf_units/tests/test_coding_standards.py#L121-L122

[pp-mo]

Closed by #461