fix: avoid using cumtrapz from scipy #242

Job	Run time
Tests on paywalled platforms (macos-13-xlarge, 3.12)	1m 44s
Tests on other platforms (windows-2019, 3.10)	5m 7s
Tox test (py38)	3m 40s
Tests on other platforms (windows-2022, 3.11)	4m 15s
Tox test (py39)	3m 43s
Tests on other platforms (windows-2022, 3.12)	4m 31s
Tox test (py310)	3m 41s
Tests on other platforms (macos-12, 3.11)	3m 54s
Tox test (py311)	3m 27s
Tests on other platforms (macos-13, 3.12)	4m 24s
Tox test (py312-cover)	6m 50s
Tox test (docs, graphviz)	1m 55s
Docker image	48s
PyPI packaging	0s
	47m 59s

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