Add tests for pickling and copying PyStructSequence objects

[serhiy-storchaka]

PyStructSequence supports pickling and copying, but there are no tests for this. There is only one trivial test which tests that __reduce__() does not raise exception or crash.

Needed explicit tests for copy.copy(), copy.deepcopy() (it is the same, but still), pickle.dumps()/pickle.loads().

cc @XuehaiPan

Linked PRs

• gh-110267: Add tests for pickling and copying PyStructSequence objects #110272
• [3.11] gh-110267: Add tests for pickling and copying PyStructSequence objects (GH-110272) #110284
• [3.12] gh-110267: Add tests for pickling and copying PyStructSequence objects (GH-110272) #110285