demote assert to debug_assert in NodePtr constructor #1619

Job	Run time
Unit tests (macos-latest, 3.7)	9m 20s
Checks	38s
Unit tests (ubuntu-latest, 3.7)	3m 5s
Wheel on macos-latest py-3.7	4m 17s
Wheel on ubuntu-latest py-3.7	2m 25s
Unit tests (windows-latest, 3.7)	5m 11s
Wheel on windows-latest py-3.7	3m 27s
coverage	7m 53s
Run fuzz targets (macos-latest, 3.7)	31m 20s
Run fuzz targets (ubuntu-latest, 3.7)	20m 23s
Upload to PyPI	23s
	1h 28m 22s

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