Use Arc::ptr_eq over .is() in SparseObservable

[Cryoris]

Summary

Follow up on #13595: a commit got lost in the final merge, this PR adds it again 🙂

Details

We should compare the inner objects of PySparseObservable, not the observable itself, since in theory multiple PySparseObservables could point to the same inner SparseObservable.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 13028907689

Details

• 20 of 20 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 88.952%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	2	93.23%

Totals
Change from base Build 13010967771:	0.02%
Covered Lines:	79694
Relevant Lines:	89592

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💛 - Coveralls

[jakelishman]

I think this is right, and that it's just the missing bits from the merged I bust up on the main merge, thanks.

If there's anything else lurking with the new form, we'll catch/fix it when we add more use of these methods.