Upgrade unittest equality method

[nkanazawa1989]

Summary

Current implementation of equality check, i.e. QiskitExperimentsTestCase.json_equiv, is not readable and scalable because it implements equality check logic for different types in a single method. This PR adds new test module test/extended_equality.py which implements new equality check dispatcher is_equivalent.

Developers no longer need to specify check_func in the assertRoundTripSerializable and assertRoundTripPickle methods unless they define custom class for a specific unittest. This simplifies unittests and improves readability of equality check logic (and test becomes more trustable).

This PR adds new software dependency in develop; multimethod

Among several similar packages, this is chosen in favor of

• its license type (Apache License, Version 2.0)
• syntax compatibility with functools.singledispatch
• support for subscripted generics in typings, e.g. Union

[coruscating]

This is a really nice upgrade and consolidation of the current comparison test code and a good use of multimethod. My comments are mainly about edge cases in the equality check.

[coruscating]

Thanks @nkanazawa1989, the fixes look good to me. I wonder if there should be an optional strict flag for is_equivalent() so that there is the option to not equate numpy arrays/tuples/lists with the same elements, or the numpy int and python int. Maybe there's no good use case for this though. Also, is your plan to wait for Qiskit/qiskit#9676 to be released before merging this?

[nkanazawa1989]

I wonder if there should be an optional strict flag

This sounds like a good idea. Done in 47dd5d5. I also added an option for numerical precision.

Also, is your plan to wait for Qiskit/qiskit#9676 to be released before merging this?

Yes. We can wait for 0.24rc1.

[coruscating]

This looks good now, thanks for adding strict_type. I'm good with merging once terra 0.24 is released and set as a requirement.