restrict depth to 1

specify sphinx version

add .readthedocs.yaml

use python3.9 to test docs

add py311 target

add xvfb-run to allowlist_externals

update docs/requirements.txt

clarify the tkinter section

display the package name only, unless clicked

update copyright

fix toc

upgrade from autodoc_default_flags to autodoc_default_options

remove the __doc__ += trick

increase toc depth to 4

fix tox py36