Add testing mode #535

Job	Run time
Build & verify package	20s
Verify dev env (ubuntu-latest)	12s
Verify dev env (windows-latest)	31s
Tests on 3.8	11s
Type-check API on 3.8	13s
Tests on 3.9	26s
Type-check API on 3.9	15s
Build docs & run doctests	11s
Tests on 3.10	12s
Type-check package	11s
Type-check API on 3.10	19s
Tests on 3.11	13s
Type-check API on 3.11	22s
Tests on 3.12	17s
Type-check API on 3.12	17s
Tests on 3.13	24s
Type-check API on 3.13	31s
Combine & check coverage	8s
Ensure everything required is passing for branch protection	4s
	5m 17s

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