MAINT: Do not run unit tests in all platforms

[oesteban]

Revert the test matrix of #234 for two reasons:

• It doesn't make a lot of sense to run all unittests for different Python versions when most of our problems will come from elsewhere as regards the platform (ANTs, data properly installed)
• We don't have a critical mass of functional code working, so tests fail for spurious installation reasons and we spend time focusing on that as opposed to getting eddymotion to a functional state.

For the time being, let's make sure it works with 3.12 and reconsider when the focus is portability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.08%. Comparing base (24ce39e) to head (ffd8067).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #240   +/-   ##
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  Coverage   64.08%   64.08%           
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  Files          18       18           
  Lines         905      905           
  Branches      117      117           
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  Hits          580      580           
  Misses        282      282           
  Partials       43       43           

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