fix: pin python version for sphinx

[michalsn]

Description
This PR pins the Python version to 3.12. Recently, we are not able to build the user guide correctly. This is because the imghdr module was removed in Python 3.13.

Running Sphinx v5.3.0

Extension error:
Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')
make: *** [Makefile:20: html] Error 2

https://github.com/codeigniter4/CodeIgniter4/actions/runs/11307496056/job/31449280998?pr=9221

More information about removing the imghdr module:
sphinx-doc/sphinx#10440

Since for some reason we are blocked on Sphinx v5 (I'm guessing because of some extensions?), pinning the Python version is the only option.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Should https://github.com/codeigniter4/CodeIgniter4/blob/develop/.github/workflows/deploy-distributables.yml#L162 be also updated to the pinned Python version? I guess yes.

[kenjis]

Should https://github.com/codeigniter4/CodeIgniter4/blob/develop/.github/workflows/deploy-distributables.yml#L162 be also updated to the pinned Python version? I guess yes.

Probably, yes.

[michalsn]

Updated.

[michalsn]

Thank you all!