Refactor optional imports #817

Job	Run time
check-package / init-store	11s
check-typing / mypy	2m 21s
check-docs / make-docs (html)	1m 34s
check-schema / schema	12s
check-docs / make-docs (doctest)	1m 6s
check-docs / make-docs (linkcheck)	1m 29s
check-package / pkg-build (ubuntu-latest)	17s
check-package / pkg-check (ubuntu-latest, 3.8)	1m 17s
check-package / pkg-check (ubuntu-latest, 3.10)	1m 5s
check-package / pkg-check (macos-13, 3.8)	1m 39s
check-package / pkg-check (macos-13, 3.10)	1m 53s
check-package / pkg-check (windows-latest, 3.8)	3m 54s
check-package / pkg-check (windows-latest, 3.10)	3m 59s
check-package / pkg-guardian	1s
	20m 58s

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