Build docs on Python 3.10

[jklaise]

• .readthedocs.yml is updated from the legacy format to the current format
• The docs on RTD are now built with ubuntu-22.04 and using Python 3.10
• Removed dependency on RTD image for GA docs build (as RTD do not recommend using them), now the GA build is run on a generic ubuntu-latest image (currently correpsonding to ubuntu-22.04 on Python 3.10 - latex builds require lots of dependencies, so easiest to use the RTD image after all
• Factored out the Python 3.10 version under a global workflow variable DEFAULT_PY_VERSION Not possible, see https://github.com/orgs/community/discussions/25295#discussioncomment-3247326

[codecov]

Codecov Report

Merging #877 (fad2de0) into master (4e3c152) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #877   +/-   ##
=======================================
  Coverage   85.29%   85.29%           
=======================================
  Files          74       74           
  Lines        8768     8768           
=======================================
  Hits         7479     7479           
  Misses       1289     1289           

Flag	Coverage Δ
macos-latest-3.10	?
ubuntu-latest-3.10	85.16% <ø> (-0.10%)	⬇️
ubuntu-latest-3.7	85.16% <ø> (+0.09%)	⬆️
ubuntu-latest-3.8	85.09% <ø> (-0.10%)	⬇️
ubuntu-latest-3.9	85.09% <ø> (ø)
windows-latest-3.9	82.83% <ø> (ø)

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