Consistent mypy version

[ADBond]

Explicitly aligning the version of mypy specified in config + used in github actions, to make it easier to get results consistent with those locally.

Results will vary between different python minor versions. Pinning this version makes it easier to align with the version running in CI, and means that it easier to avoid accidentally introducing typing features not available until newer python versions. 3.10 seems a reasonable minimum, ahead of 3.9's retirement later in the year.

[codecov-commenter]

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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2695   +/-   ##
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  Coverage          ?   80.78%           
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  Files             ?      106           
  Lines             ?     8982           
  Branches          ?        0           
=========================================
  Hits              ?     7256           
  Misses            ?     1726           
  Partials          ?        0           

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