Fix pickle and repr for Duration. (backport #14174)

[mergify]

Summary

Adds missing Python pickle support for the new Duration type added in Qiskit 2.0.

Details and comments

To get this working, I needed to manually register the PyO3 generated Duration_xx Python classes at the module level, since pickle looks for them during deserialization. I'm not sure if there's a better way, but I'm open to suggestions if you can think of something cleaner. Pickle doesn't have explicit support in PyO3, so we're lucky to have any working solution here.

While fixing this, I also noticed that __repr__ was defined inside an impl block that wasn't decorated with #[pymethods], meaning it wasn't actually being used before. That's fixed here too.

Fixes #14169

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This is an automatic backport of pull request #14174 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 14330294605

Details

• 21 of 26 (80.77%) changed or added relevant lines in 2 files are covered.
• 23 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.088%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/circuit/src/lib.rs	15	20	75.0%

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.23%
crates/qasm2/src/lex.rs	4	91.73%
crates/qasm2/src/parse.rs	18	96.68%

Totals
Change from base Build 14316915198:	-0.02%
Covered Lines:	72872
Relevant Lines:	82726

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💛 - Coveralls