feat: separate span power from tx power #306

Job	Run time
Tests on other platforms (windows-2019, 3.10)	3m 1s
Tox test (py38)	2m 38s
Tests on other platforms (windows-2022, 3.11)	3m 14s
Tests on paywalled platforms	0s
Tox test (py39)	2m 29s
Tests on other platforms (windows-2022, 3.12)	3m 47s
Tox test (py310)	2m 28s
Tests on other platforms (macos-12, 3.11)	2m 30s
Tox test (py311)	2m 33s
Tests on other platforms (macos-13, 3.12)	2m 50s
Tox test (py312-cover)	5m 16s
Tox test (docs, graphviz)	1m 53s
PyPI packaging	0s
Docker image	0s
	32m 39s

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