change import of Backend

[Edoardo-Pedicillo]

This is a small fix, to be compatible with Qibo 0.2.8 (qiboteam/qibo@18d59d7)

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.
• Compatibility with Qibo modules (Please edit this section if the current pull request is not compatible with the following branches).
  • Qibo: master
  • Qibolab: main
  • Qibolab_platforms_qrc: main

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.25%. Comparing base (7a422fc) to head (c192841).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
- Coverage   97.28%   97.25%   -0.04%     
==========================================
  Files         109      109              
  Lines        8043     8044       +1     
==========================================
- Hits         7825     7823       -2     
- Misses        218      221       +3     

Flag	Coverage Δ
unittests	97.25% <100.00%> (-0.04%)	⬇️

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

Files	Coverage Δ
src/qibocal/auto/runcard.py	95.91% <100.00%> (+0.08%)	⬆️
src/qibocal/auto/transpile.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[andrea-pasquale]

@Edoardo-Pedicillo we have released qibo today, as soon as you update the lock you should be able to merge this PR today.