Move the jupytext-config helpers to a subpackage, and add tests #90

Job	Run time
pre-commit / pre-commit	33s
pre-commit / lint-extension	2m 6s
codeql / codeql	2m 57s
test-ui / test-ui	3m 21s
test-unit-functional-integration / test-pip (3.11, unit)	2m 42s
test-pip / test-pip (3.8, ~=3.0)	9m 43s
test-unit-functional-integration / test-pip (3.11, functional)	3m 46s
test-conda / test-conda (ubuntu-latest, 3.11)	11m 13s
test-pip / test-pip (3.9, ~=3.0)	9m 25s
test-unit-functional-integration / test-pip (3.11, integration)	3m 25s
test-conda / test-conda (macos-latest, 3.11)	19m 2s
test-pip / test-pip (3.10, ~=3.0)	9m 51s
test-unit-functional-integration / test-pip (3.11, external)	6m 22s
test-conda / test-conda (windows-latest, 3.11)	14m 29s
test-pip / test-pip (3.11, ~=3.0)	9m 28s
test-pip / test-pip (3.12-dev, ~=3.0)	11m 49s
test-pip / test-pip (3.x, ~=2.0)	11m 47s
test-pip / test-pip (3.x)	11m 30s
test-pip / test-pip (3.x, true)	9m 36s
test-pip / test-pip (3.x, true)	12m 20s
build / build	2m 59s
	2h 48m 24s

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