unpin sphinx to make rtd happy #136

Job	Run time
precommit / Load tox environments	4s
test / Load tox environments	6s
precommit / pre-commit (ubuntu-latest)	33s
test / py38 (macos-latest)	4m 51s
test / py310 (macos-latest)	6m 41s
test / py38 (windows-latest)	3m 41s
test / py310 (windows-latest)	4m 4s
test / py38 (ubuntu-latest)	2m 6s
test / py39 (ubuntu-latest)	2m 15s
test / py310 (ubuntu-latest)	2m 13s
test_full_database / Load tox environments	7s
docs / Load tox environments	8s
docs / build_docs (ubuntu-latest)	2m 12s
test_full_database / py310 (ubuntu-latest)	6m 43s
publish / Build source and wheel distribution	4m 41s
	40m 25s

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