Fix test_entropy for cupy backend

[renatomello]

Checklist:

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

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c5b8d96) 100.00% compared to head (c72d400) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #912   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         6578      6578           
=========================================
  Hits          6578      6578           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/qibo/quantum_info/metrics.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.