Add qpy deprecation warning class

[ElePT]

Summary

This PR proposes a new QPYLoadingDeprecatedFeatureWarning class inspired in numpy's VisibleDeprecationWarning with the purpose of ensuring that deprecation messages reach the user even if they are raised from variable points in the call stack. The intended use case are qpy loading functions that can be called recursively (hence the name).

Details and comments

The original issue was found during the review of #11257

Note: I have tested that the warning is visible using the example from #11257. I dumped a schedule with a complex amplitude using qpy v.5 (qiskit-terra 0.21) and tried loading it with the latest qpy version. The warning wouldn't surface with the usual DeprecationWarning class but it did with QPYLoadingDeprecatedFeatureWarning:

[coveralls]

Pull Request Test Coverage Report for Build 7274445496

• 4 of 4 (100.0%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 87.555%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.41%

Totals
Change from base Build 7266513358:	0.03%
Covered Lines:	59161
Relevant Lines:	67570

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💛 - Coveralls

[jakelishman]

Don't forget to add a documentation line for the new warning in the QPY API docs (you can use autoexception, because Python makes tons of sense).

[TsafrirA]

Do you plan on having this limited to DeprecationWarning class? It will be helpful to be able to raise other types of warnings.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @mtreinish
• @nkanazawa1989

[ElePT]

Do you plan on having this limited to DeprecationWarning class? It will be helpful to be able to raise other types of warnings.

What other types of warnings are you thinking about @TsafrirA? This seemed to be an issue particularly for DeprecationWarning, but I think that most of the exceptions we work with should normally surface without the need for a new class.

[TsafrirA]

What other types of warnings are you thinking about

I am planning to replace the deprecation warning with a user warning once the process of #11257 is completed. That shouldn't be a problem?

[ElePT]

What other types of warnings are you thinking about

I am planning to replace the deprecation warning with a user warning once the process of #11257 is completed. That shouldn't be a problem?

You can replace DeprecationWarning with QPYLoadingDeprecatedFeatureWarning in #11257 without issues. The only thing to keep in mind is that to catch it, you must specify the QPYLoadingDeprecatedFeatureWarning type.

[TsafrirA]

I didn't explain myself well.

Once we remove the support which is deprecated in #11257, pulses loaded from old QPY files will be altered. I was planning of issuing a user warning at that point to alert the user to the change. Do we then need a similar mechanism for a user warning?

[jakelishman]

This looks sensible to me (except for why Python calls warnings exceptions), thanks!