Revisit pickling logic for TraitListObject

This is a reminder to myself to revisit the pickling infrastructure for `TraitListObject` at some point before the 6.1.0 release. In particular:

- we don't have good tests for pickling and unpickling; there are some indirect tests, but it would be good to have direct unit tests in `test_trait_list_object`
- with the right unpickling setup, it shouldn't be necessary to verify that `self.trait` exists when validating the length
- I'd like to understand better the need for the `object` and `trait` manipulations in `__getstate__` and `__setstate__`.