Fix bug with `md2po --no-obsolete` #958

Job	Run time
Lint	4m 41s
Test (3.8, ubuntu-latest)	38s
Build documentation	25s
Test (3.8, macos-latest)	1m 32s
Test (3.8, windows-latest)	1m 42s
Test (3.9, ubuntu-latest)	36s
Test (3.9, macos-latest)	26s
Test (3.9, windows-latest)	1m 14s
Test (3.10, ubuntu-latest)	33s
Test (3.10, macos-latest)	32s
Test (3.10, windows-latest)	1m 34s
Test (3.11, ubuntu-latest)	30s
Test (3.11, macos-latest)	47s
Test (3.11, windows-latest)	1m 39s
Build package	14s
Build standalone (ubuntu-latest)	39s
Build standalone (macos-14)	19s
Build standalone (macos-13)	34s
Build standalone (windows-latest)	1m 38s
Build final distribution	0s
Release	0s
Upload to PyPI	0s
	20m 13s

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