Bump sphinx version to 6.x.x

[michaelweinold]

sphinx-design currently still requires sphinx < 6:

• support sphinx 6 executablebooks/sphinx-design#118

myst-nb currently still requires sphinx < 6 (and myst-parser ~ 0.18):

• RFE: please provide myst-parser 0.19.x and sphinx 6.x support executablebooks/MyST-NB#476
• Bump version MyST-parser // Sphinx >= 6 executablebooks/MyST-NB#493
• allow for sphinx 6.0 executablebooks/MyST-NB#466
• UPGRADE: myst-parser 1.0 executablebooks/MyST-NB#479

readthedocs.org supports pinning Sphinx to specific verions:

If you want to pin Sphinx to a specific version, use a requirements.txt or environment.yml file (see Requirements file and conda.environment).

cf. also:

• Ability to pin Sphinx and friends in a conda environment readthedocs/readthedocs.org#3829

[michaelweinold]

Trying to upgrade to sphinx=6.2.1 throws the error (only on readthedocs):

Warning: you have pip-installed dependencies in your environment file, but you do not list pip itself as one of your conda dependencies.  Conda may not use the correct pip to install your packages, and they may end up in the wrong place.  Please add an explicit pip dependency.  I'm adding one for you, but still nagging you.
Encountered problems while solving:
  - package sphinx-6.2.1-pyhd8ed1ab_0 requires docutils >=0.18.1,<0.20, but none of the providers can be installed

Compare the open issue:

• Build fails for sphinx=6.2.1, claiming "requires docutils >=0.18.1,<0.20, but none of the providers can be installed" readthedocs/readthedocs.org#10477