DEP: drop undeclared runtime dependency on `setuptools` #12

Job	Run time
tests / Load tox environments	4s
publish / Build source and wheel distribution	1m 16s
tests / codestyle (ubuntu-latest)	17s
tests / py38-test-mpl33 (ubuntu-latest)	36s
tests / py39-test-mpl34 (ubuntu-latest)	39s
tests / py39-test-mpl35 (ubuntu-latest)	32s
tests / py310-test-mpl36 (ubuntu-latest)	29s
tests / py310-test-mpl37 (ubuntu-latest)	33s
tests / py311-test-mpldev (ubuntu-latest)	1m 32s
tests / py38-test-mpl33 (windows-latest)	1m 15s
tests / py39-test-mpl34 (macos-12)	1m 3s
tests / py39-test-mpl35 (windows-latest)	1m 1s
tests / py310-test-mpl36 (macos-latest)	35s
tests / py310-test-mpl37 (windows-latest)	1m 35s
tests / py311-test-mpldev (windows-latest)	3m 22s
tests / py311-test-mpl38-image (ubuntu-latest)	9s
	14m 58s

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