pinning of python to 3.10 or 3.11 for RTFD

[ReimarBauer]

The environment is created by

mamba env create --quiet --name stable --file docs/environment.yml

the output by

python -m sphinx -T -b html -d _build/doctrees -D language=en . $READTHEDOCS_OUTPUT/html

We don't have xstatic for 3.12 yet, and there are more problems

Running Sphinx v7.2.6

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/checkouts/stable/docs/conf.py", line 81, in <module>
    import mslib.mswms.wms
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/checkouts/stable/docs/../mslib/mswms/wms.py", line 44, in <module>
    from future import standard_library
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/future/standard_library/__init__.py", line 65, in <module>
    import imp
ModuleNotFoundError: No module named 'imp'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/cmd/build.py", line 293, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/application.py", line 211, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/config.py", line 181, in read
    namespace = eval_config_file(filename, tags)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/config.py", line 371, in eval_config_file
    raise ConfigError(msg % traceback.format_exc()) from exc
sphinx.errors.ConfigError: There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/checkouts/stable/docs/conf.py", line 81, in <module>
    import mslib.mswms.wms
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/checkouts/stable/docs/../mslib/mswms/wms.py", line 44, in <module>
    from future import standard_library
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/future/standard_library/__init__.py", line 65, in <module>
    import imp
ModuleNotFoundError: No module named 'imp'


Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/checkouts/stable/docs/conf.py", line 81, in <module>
    import mslib.mswms.wms
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/checkouts/stable/docs/../mslib/mswms/wms.py", line 44, in <module>
    from future import standard_library
  File "/home/docs/checkouts/readthedocs.org/user_builds/mss/conda/stable/lib/python3.12/site-packages/future/standard_library/__init__.py", line 65, in <module>
    import imp
ModuleNotFoundError: No module named 'imp'