Fix deserialization of crosstalk_matrix

[andrea-pasquale]

This PR fixes the crosstalk_matrix deserialization which was broken after switching from yaml to json.
I found this bug while testing qiboteam/qibocal#684.
If someone knows a more elegant solution let me know.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1c8650b) 65.10% compared to head (e717b9f) 65.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #813      +/-   ##
==========================================
+ Coverage   65.10%   65.13%   +0.02%     
==========================================
  Files          50       50              
  Lines        6027     6031       +4     
==========================================
+ Hits         3924     3928       +4     
  Misses       2103     2103              

Flag	Coverage Δ
unittests	65.13% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[alecandido]

If someone knows a more elegant solution let me know.

I know: stop using non-integer IDs for serialization and move the matrix out of the single qubit :P