Tests

test: Turn `tests` into a package #2142

Job	Run time
Build doc	1m 2s
Run (most) tests with default install	1m 9s
Run mypy	1m 10s
Run tests with full install (3.11, macOS-latest)	1m 2s
Run tests with full install (3.11, windows-latest)	1m 45s
Run tests with full install (3.11, ubuntu-latest)	1m 20s
Run tests with full install (3.10, ubuntu-latest)	1m 17s
Run tests with full install (3.12, ubuntu-latest)	2m 44s
Run tests with full install (3.12, macOS-latest)	1m 1s
Run tests with full install (3.10, windows-latest)	1m 46s
Run tests with full install (3.10, macOS-latest)	1m 18s
Run tests with full install (3.13, ubuntu-latest)	1m 23s
Run tests with full install (3.13, windows-latest)	1m 45s
Run tests with full install (3.14, macOS-latest)	1m 33s
Run tests with full install (3.14, ubuntu-latest)	1m 31s
Run tests with full install (3.14, windows-latest)	2m 43s
Run tests with full install (3.12, windows-latest)	1m 33s
Run tests with full install (3.13, macOS-latest)	1m 8s
	27m 10s

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