Tests

switch to tomli for toml fallback #550

Job	Run time
Run important tests (3.7 on ubuntu-latest)	4s
Run important tests (pypy-3.7 on ubuntu-latest)	3m 21s
Run important tests (pypy-3.10 on ubuntu-latest)	2m 40s
Run important tests (3.12 on macos-latest)	49s
Run important tests (3.12 on windows-latest)	2m 52s
Run all tests (3.12 on ubuntu-latest)	6m 4s
Tests done	1s
	15m 51s

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