PersistentDict: thread safety #1321

Job	Run time
Mypy	22s
Pylint	36s
Documentation	29s
Tests for downstream project loopy	3m 56s
Pytest on Py3.8 ubuntu-latest	50s
Tests for downstream project pytato	5m 36s
Pytest without Numpy	1m 0s
Pytest on Py3.8 macos-latest	1m 19s
Pytest on Py3.9 ubuntu-latest	50s
Ruff	5s
Pytest on Py3.9 macos-latest	57s
Validate CITATION.cff	11s
Pytest on Py3.x ubuntu-latest	55s
Pytest on Py3.x macos-latest	1m 4s
Pytest on Pypypy3.8 ubuntu-latest	1m 33s
Pytest on Pypypy3.8 macos-latest	5m 4s
	24m 47s

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