Adopt Py3.8+ features: Drop workaround in PicklePersistence

[Bibo-Joshi]

With dropping python 3.7 it's possible to remove a py-37-specific workorund in the PicklePersistence implementation. More details can be found here. We should revisit that implementation and remove all python-3.7 specific code. The corresponding test cases also needs to be adapted to drop any python 3.7 related clauses.

A PR in this direction is very welcome. As long as #3728 is open, it should be based on the drop-py-37 branch. If you would like to send a PR, please be sure to read our contribution guide and leave a short comment so that we can assign you.