PersistentDict: thread safety #1326

Job	Run time
Pylint	44s
Ruff	5s
Documentation	32s
Tests for downstream project loopy	4m 6s
Mypy	25s
Tests for downstream project pytato	5m 33s
Pytest on Py3.8 ubuntu-latest	48s
Pytest on Py3.8 macos-latest	1m 58s
Pytest without Numpy	1m 1s
Pytest on Py3.9 ubuntu-latest	50s
Validate CITATION.cff	15s
Pytest on Py3.9 macos-latest	56s
Pytest on Py3.x ubuntu-latest	53s
Pytest on Py3.x macos-latest	50s
Pytest on Pypypy3.8 ubuntu-latest	1m 36s
Pytest on Pypypy3.8 macos-latest	4m 14s
	24m 46s

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