Move the jupytext-config helpers to a subpackage, and add tests #85

Job	Run time
pre-commit / pre-commit	25s
pre-commit / lint-extension	2m 26s
codeql / codeql	3m 4s
test-ui / test-ui	4m 35s
test-unit-functional-integration / test-pip (3.11, unit)	2m 55s
test-unit-functional-integration / test-pip (3.11, functional)	3m 31s
test-unit-functional-integration / test-pip (3.11, integration)	3m 12s
test-conda / test-conda (ubuntu-latest, 3.11)	10m 32s
test-unit-functional-integration / test-pip (3.11, external)	6m 15s
test-pip / test-pip (3.8, ~=3.0)	9m 25s
test-conda / test-conda (macos-latest, 3.11)	21m 9s
test-conda / test-conda (windows-latest, 3.11)	13m 23s
test-pip / test-pip (3.9, ~=3.0)	9m 29s
test-pip / test-pip (3.10, ~=3.0)	9m 26s
test-pip / test-pip (3.11, ~=3.0)	9m 10s
test-pip / test-pip (3.12-dev, ~=3.0)	11m 42s
test-pip / test-pip (3.x, ~=2.0)	11m 33s
test-pip / test-pip (3.x)	11m 48s
test-pip / test-pip (3.x, true)	10m 46s
test-pip / test-pip (3.x, true)	12m 10s
build / build	2m 54s
	2h 49m 50s

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