Deprecate Python 3.7 support

[mtreinish]

Summary

This commit deprecates Python 3.7 support. During the deprecation window we will continue to fully support Python 3.7 and test it in CI, but it will emit a warning that we will be removing support for 3.7 in the future to inform users to upgrade to Python 3.8 or newer. The removal will occur during the qiskit-terra 0.25.0 release as it is the first release scheduled to occur after the upstream Python EoL date in June 2023. [1]

Details and comments

[1] https://devguide.python.org/versions/

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[coveralls]

Pull Request Test Coverage Report for Build 3348648978

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.498%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/init.py	1	2	50.0%

Totals
Change from base Build 3348648561:	0%
Covered Lines:	62201
Relevant Lines:	73612

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💛 - Coveralls

[jakelishman]

I wouldn't be opposed to us considering doing the removal in Terra 0.24, if that's consistent with our policies (I can't remember), on the basis that Terra 0.24 will still be live while Python 3.7 goes EOL - if we wait til the version after, we officially have to support Python 3.7 after it's done.

I'm also keen to drop Python 3.7 asap really, because the number of work-arounds we have in Terra for pre-Python-3.8 is already big, and it's only going to get bigger over the next 9 months til 0.25.

[mtreinish]

Well, it'd violate the deprecation policy if we removed it during 0.24.0 because there isn't an intermediate release between the initial deprecation and the release removal. But we'll be able to remove it from main as soon as we branch stable/0.24 so realistically the burden for development is only the next ~6 months.

[jakelishman]

Yeah, violating the policy is no good. I wish I'd remembered about it before we did 0.22, because I would have wanted to discuss it then.

For maintenance: it's maintenance of the stable branch I'm concerned about - if something breaks in backport/user testing, we may well still need to be putting out more workarounds for an EOL Python into our latest of the software right up til the 0.25 release. But given that we missed the window to discuss it before 0.22, it's a fairly moot point now.

[jakelishman]

I'm very much looking forwards to seeing the back of Python 3.7 (although that does feel a bit strange for me to say, since it was the version I used for most of my PhD!). I think it's good to get this merged in sooner rather than later, so people who care about avoiding the deprecation warnings (and test against main Terra) have longer to react.