Let readthedocs manage the sphinx installation.

Also switch to the = syntax to pull the latest bugfix release 3.11.*.
Problems with the bulleted lists should have been fixed: readthedocs/readthedocs.org#8252
But conda environments don't work well when the users specifies sphinx and sphinx_rtd_theme: readthedocs/readthedocs.org#3829

sphinx-doc/environment.yml

@@ -1,9 +1,7 @@
 channels:
   - conda-forge
 dependencies:
-  - python==3.10
-  - sphinx
-  - sphinx_rtd_theme
+  - python=3.11
   - ipython
   - ipython_genutils
   - numpy