PersistentDict: thread safety #1324

Job	Run time
Pylint	41s
Mypy	23s
Documentation	38s
Tests for downstream project loopy	4m 6s
Pytest on Py3.8 ubuntu-latest	53s
Tests for downstream project pytato	5m 36s
Pytest without Numpy	55s
Pytest on Py3.8 macos-latest	1m 15s
Ruff	5s
Pytest on Py3.9 ubuntu-latest	47s
Validate CITATION.cff	17s
Pytest on Py3.9 macos-latest	53s
Pytest on Py3.x ubuntu-latest	53s
Pytest on Py3.x macos-latest	1m 11s
Pytest on Pypypy3.8 ubuntu-latest	1m 27s
Pytest on Pypypy3.8 macos-latest	4m 4s
	24m 4s

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