Drop Python 3.7 support

[woodsp-ibm]

Summary

CI nightly has been failing now for a while under 3.7 around mypy and rustworkx - the latter got updated to 0.13.1 and the constraint we had no longer worked. While I could have amended constraint, as Qiskit has now dropped support for 3.7 this drops it from Nature too, rendering the rustworkx constraint mute (which I removed), and means CI should now pass.

Details and comments

A reno notes the upgrade needed for the user.

• Once this passes we should amend the CI branch rules accordingly.

This should also allow #1222 to go forwards once merged.

I had meant to look for sys version checks and forgot (now done) - CI flagged a line no longer being tested and it turns out it was a version check. So I just removed this and what was needed for 3.7. This was protocols.py - I mused over changing the eigenstate result not to be a string - but there are other places where this could be done so Iet it be. I had noticed as the string was missing a period - which I fixed; I let that be the extent of the change otherwise.

[coveralls]

Pull Request Test Coverage Report for Build 5858396532

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 87.874%

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Totals
Change from base Build 5445189731:	0.5%
Covered Lines:	9812
Relevant Lines:	11166

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💛 - Coveralls

[mrossinek]

Thanks a lot, Steve! 👍