KeyBuilder: allow hashing of classes that define update_persistent_hash #1026

Job	Run time
Documentation	26s
Flake8	15s
Mypy	23s
Tests for downstream project loopy	4m 4s
Tests for downstream project pytato	5m 20s
Pylint	29s
Pytest on Py3.8	25s
Pytest without Numpy	18s
Pytest on Py3.9	23s
Validate CITATION.cff	7s
Pytest on Py3.x	30s
Pytest on Pypypy3.8	46s
	13m 26s

Provide feedback