drop support for Python 3.7 #520

Job	Run time
coverage	6m 5s
typecheck	1m 50s
lint	2m 50s
docs	3m 38s
test (macos-latest, 3.7)	3m 9s
test (macos-latest, 3.8)	9m 0s
test (macos-latest, 3.9)	8m 54s
test (macos-latest, 3.10)	9m 25s
test (macos-latest, 3.11)	10m 15s
test (ubuntu-latest, 3.7)	2m 34s
test (ubuntu-latest, 3.8)	6m 10s
test (ubuntu-latest, 3.9)	5m 43s
test (ubuntu-latest, 3.10)	6m 0s
test (ubuntu-latest, 3.11)	6m 0s
test (windows-latest, 3.7)	2m 55s
test (windows-latest, 3.8)	9m 44s
test (windows-latest, 3.9)	7m 1s
test (windows-latest, 3.10)	7m 25s
Rust unit tests (ubuntu-latest)	5m 58s
Rust unit tests (macos-latest)	5m 34s
Rust unit tests (windows-latest)	6m 19s
Rustfmt	17s
	2h 6m 46s

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