add pytest-freethreaded plugin

[thepabloaguilar]

I have made things!

This PRs introduces a new pytest plugin called pytest-freethreaded which runs each test multiple times using threads and potentially tell us what are the areas we need to look for non thread safety things.

I've added a constraint to only install this plugin for Python 3.14+ versions!

Note

This PR is not intended to be merge, it's more a exploration. The fixes that would come from the results here will be fixed in separated PRs and then we rebase this branch to get the fixed stuff

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made

Related issues

Refs #55

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5c7d315) to head (c73fd22).
⚠️ Report is 339 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #416   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          195       195           
  Lines         6776      6776           
  Branches       213       213           
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  Hits          6776      6776           

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[thepabloaguilar]

I've looked at some errors, the errors are happening because the plugin does not run with asyncio support 😞

[thepabloaguilar]

Ok, in fact it's just because of the pytest plugin initialization order to be arbitrary

[sobolevn]

It now has a lot of merge conflicts :(