Fixed #23594 -- Fixed deepcopy on ErrorList.

Thanks Troy Grosfield for the report and Tim Graham for the tests.
FWIW, I usually tell contributors not to include ticket numbers in the actual code since it can quickly clutter things up if we did that for every change. Do you think that this is sufficiently obscure that the comments can't fully describe the issue? I'd be in favor of moving some of this info to the test explaining that tests are for `ErrorDict.__reduce_ex__`.