feat: add support for Python 3.14 and Django 6.0

[Adiorz]

Fixes #86

[zsoldosp]

@Adiorz thanks for the pull. I'll take a look at this later this week. thanks for the patience!

[zsoldosp]

@Adiorz thanks again for the pull !

Jut two requests please

• Django 4.2 support - please don't drop it, as it still has extended support until
  April 2026 https://www.djangoproject.com/download/#supported-versions I think dropping the python
  3.9 version support is still a valid change, as it's EOL
• Django 5.2 also supports 3.14 python now (tox.ini's envlist)

And a question about rst2html.py vs docutils - it's not documented in the README/Makefile, but the
reason for this line in the Makefile was to ensure the pypi publishing process doesn't fail,
as the README.md is used for the package description. In early releases for this or other
packages, we've had issues, so this sanity check was added to CI. Do you think/know
whether the docutils command catches the same class of errors?

thanks!

[Adiorz]

And a question about rst2html.py vs docutils - it's not documented in the README/Makefile, but the
reason for this line in the Makefile was to ensure the pypi publishing process doesn't fail,
as the README.md is used for the package description. In early releases for this or other
packages, we've had issues, so this sanity check was added to CI. Do you think/know
whether the docutils command catches the same class of errors?

@zsoldosp thanks for the review

Django 4.2 support is reverted back now. Django 5.2 and Python 3.14 pair is also added now.

The two commands are functionally equivalent for the purpose of reStructuredText syntax validation in CI, but I reverted it to rst2html (no .py starting from version 0.21 of docutils according to https://docutils.sourceforge.io/RELEASE-NOTES.html#release-0-21-2024-04-09) to be completely safe here