build: drop python3.7 add 3.13 in CI, replace poetry action

[bigpick]

Info

1. Python 3.7 is EOL as of almost 2 years ago, so increasing minimum supported version to 3.8
2. 3.13 is also released now, so adding that as well
3. Removed the poetry action in favor of a set of pipx+poetry env use commands, to remove dependency on external outdated action.
   • Install with pipx before python setup works bc runners come with it already
   • poetry env ... is needed in order to leverage the specific python version otherwiise it will fall to the one that was default on the runner that was used to install with pipx, which is not desired.
4. Added a Python version badge for 3.8 - 3.13 to the list of badges on the README

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4.8 is also EOL, so should really update to 3.9 - but for now at least the GitHub runners still widely support 3.8 so leaving it there (the same can't be said for 3.7).

[bigpick]

OK - checks all pass now