Fix circuit copy modifying the original queue

[stavros11]

Fixes #783.

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 (d9a2dda) 100.00% compared to head (a008cad) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #805   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           47        47           
  Lines         5733      5728    -5     
=========================================
- Hits          5733      5728    -5     

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/models/circuit.py	100.00% <100.00%> (ø)

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