Update minimum python version (3.9) in pyproject.toml and docs

[hikinggrass]

Changes included in this PR

• Update minimum python version (3.9) in pyproject.toml and docs

Current behavior

• The docs state that a minimum of python 3.7 is supported
• The classifiers in pyproject.toml state that a minimum of python 3.8 is supported
• The tool.poetry.dependencies requires python 3.11
• This results in the latest release (2.1.0) on pypi not being listed in pip index versions ocpp and not being available to install (by default) ob Ubuntu 22.04, which ships python 3.10

New behavior

• A consistent minimum python version of 3.9 is used (lower than that did not work without further changes)

Impact

• Installing is possible again on older python versions

Checklist

1. Does your submission pass the existing tests?
2. Are there new tests that cover these additions/changes?
3. Have you linted your code locally before submission?

[hikinggrass]

I just saw the commit that added sphinx 8.x as a dependency mentioning python 3.11 as a minimum for development going forward. Then this should probably just be reflected in the docs. Looks like the CI Tests for 3.8 etc. currently also use 3.12 (eg https://github.com/mobilityhouse/ocpp/actions/runs/16022303628/job/45201948389 ) maybe that's why this wasn't detected yet