Bump version to 0.3.1 and prepare for release #183

Job	Run time
audit	5m 5s
build-and-test (ubuntu-latest, stable)	1m 41s
formatting	10s
linting	32s
build-and-test (ubuntu-latest, beta)	1m 35s
build-and-test (macos-latest, stable)	1m 45s
build-and-test (macos-latest, beta)	1m 55s
build-and-test (windows-latest, stable)	2m 54s
build-and-test (windows-latest, beta)	2m 26s
build-and-test (ubuntu-latest, nightly)	1m 33s
build-and-test (ubuntu-latest, 1.70.0)	1m 21s
	20m 57s

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