KeyBuilder: allow hashing of classes that define update_persistent_hash #1025

Job	Run time
Flake8	14s
Documentation	32s
Mypy	23s
Tests for downstream project loopy	4m 3s
Pylint	30s
Tests for downstream project pytato	5m 7s
Pytest on Py3.8	23s
Pytest without Numpy	18s
Pytest on Py3.9	25s
Validate CITATION.cff	7s
Pytest on Py3.x	27s
Pytest on Pypypy3.8	45s
	13m 14s

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