Align Python versions througout MALA

[DanielKotik]

At the moment we use/mention different Python versions at several places, e.g.

• mala/docs/source/install/installing_mala.rst

  Line 8 in e6e831b

  version it has been tested with successfully is Python ``3.10.4``.

• mala/.github/workflows/publish-to-pypi.yml

  Line 25 in e6e831b

  python-version: '3.8'

• mala/.github/workflows/gh-pages.yml

  Line 22 in e6e831b

  python-version: '3.12'

• mala/install/mala_cpu_base_environment.yml

  Line 6 in e6e831b

  - python>=3.6, <3.9

• mala/install/mala_cpu_environment.yml

  Line 116 in e6e831b

  - python=3.8.15

We should ensure to support at least one Python version that suits the mala package and building of the documentation at the time. In a next step we might think about supporting multiple Python version and run pipelines for to cover these different versions, see for example: https://docs.github.com/en/actions/using-jobs/using-a-matrix-for-your-jobs

Bonus: add badge of supported Python verions

[elcorto]

Also this:

mala/.pre-commit-config.yaml

Line 13 in 0fed9cc

language_version: python3.12

[elcorto]

I also suggest to have the mininmal (and maximal if needed, see #559) supported version in pyproject.toml, or setup.py if we continue using it.