Add support and testing for Python 3.10.

[tobiasraabe]

Changes

Add support for Python 3.10.

This is currently blocked since pony is not ready for the new Python version, but they are making big progress: https://github.com/ponyorm/pony/issues/598.

[codecov]

Codecov Report

Merging #153 (393df1b) into main (757e617) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #153   +/-   ##
=======================================
  Coverage   92.95%   92.95%           
=======================================
  Files          75       75           
  Lines        6830     6830           
=======================================
  Hits         6349     6349           
  Misses        481      481           

Flag	Coverage Δ
end_to_end	78.71% <ø> (ø)
integration	41.85% <ø> (ø)
unit	70.07% <ø> (ø)

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