Temporary fix for intermittent failures of test_execute_with_pauli_twirling

[Misty-W]

Description

Temporary fix for intermittent failures of test_execute_with_pauli_twirling: adjust allowed upper bound of expectation value from 0.4 to 0.5.

Longer-term fix (issue #1914) is to test PT in a way that doesn't treat it as a mitigation technique and remove the execute_with_pauli_twirling function while retaining mitigate_executor for composing Pauli Twirling with error mitigation techniques in the Mitiq workflow.

Fixes #1858.

---

License

• I license this contribution under the terms of the GNU GPL, version 3 and grant Unitary Fund the right to provide additional permissions as described in section 7 of the GNU GPL, version 3.

Before opening the PR, please ensure you have completed the following where appropriate.

• I added unit tests for new code.
• I used type hints in function signatures.
• I used Google-style docstrings for functions.
• I updated the documentation where relevant.
• Added myself / the copyright holder to the AUTHORS file

[github-actions]

👈 Launch a binder notebook on branch unitaryfund/mitiq/1858-test-of-execute_w_pt-fails-intermittently

[codecov]

Codecov Report

Merging #1931 (73c82a0) into master (6761302) will increase coverage by 98.47%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1931       +/-   ##
===========================================
+ Coverage        0   98.47%   +98.47%     
===========================================
  Files           0       79       +79     
  Lines           0     3681     +3681     
===========================================
+ Hits            0     3625     +3625     
- Misses          0       56       +56     

see 79 files with indirect coverage changes

[natestemen]