Backport PR #61770 on branch 2.3.x (BUG: Fix unpickling of string dtypes of legacy pandas versions)
Backport PR #61770: BUG: Fix unpickling of string dtypes of legacy pandas versions
I am adding an ignore for a new numpy deprecation warning. Although it also happens to occur in a pickle test, it is not actually related to the changes in this PR. It just started happening now because the linked numpy PR was merged a few days ago (the same failures can be seen on the 2.3.x branch). 
And it is also only for a test that is already removed in the main branch (testing py2 compat in pickle), so just ignoring here seems fine