add support for python 3.13

[branchv]

This PR contains a couple of fixes for python 3.13:

• cheetah3 has been renamed to ct3, with the former no longer being updated and broken on 3.13: Release Python 3.13 support CheetahTemplate3/cheetah3#63
• Invalid escape sequences now raise a SyntaxWarning on 3.12+: https://docs.python.org/3/whatsnew/3.12.html#other-language-changes
• While here, migrate mostly to pyproject.toml (the modern replacement for setup.py favoring static metadata). Now only the dynamic version remains in setup.py.

Fixes #681

Checklist

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[freyes]

Hi, thanks for this PR, this is something we must definitively want to have, I just left one comment inline, which prompts me to think that maybe we would need to cut a new major release (charm-tools 4.x ) to drop older versions of python

[freyes]

this is an aspect that makes be concerned, how compatible this is with older versions of python, do you know what versions can we support with pyproject.toml ?

[branchv]

Yea compatibility is determined by the build backend. If you want to stick with setuptools (the only setuptools-specific feature this currently uses is setting the dynamic version), they added initial support for pyproject.toml in 61.0.0, which supports >=3.7 and later stabilized in 68.1.0, which supports >=3.8

If you need to support even older pythons, you can pick another build backend (flit for example supported >=3 for a while but it can only read version from a static file)

What python versions do you need to support? I will note that most core packaging projects use only pyproject.toml themselves these days, including pip, setuptools, packaging, etc

[freyes]

3.8 (focal) is fine, that's the oldest we support :-)

[branchv]

Sounds good, added a requires-python to make that support explicit

[freyes]

Add 3.13 to the testing matrix please, - https://github.com/juju/charm-tools/blob/master/.github/workflows/tests.yml#L16 - also we need a maintainer to allow this PR to run the CI

[daeho-ro]

What about 3.14 ? ruamel.yaml need to be updated for 0.18.15.