Add tests for `str` formatting of `Price` and `Power` types #285

Job	Run time
Build distribution packages	14s
Test with nox (ubuntu-20.04, 3.11, ci_checks_max)	1m 10s
Test with nox (ubuntu-20.04, 3.11, pytest_min)	43s
Cross-arch tests with nox	0s
Test documentation website generation	39s
Cross-arch tests with nox	0s
Test package installation in different architectures	1m 29s
Test with nox	1s
Publish documentation website to GitHub pages	0s
Create GitHub release	0s
Publish packages to PyPI	0s
	4m 16s

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