Allow --mpl-generate-summary without requiring --mpl #277

Job	Run time
test / Load tox environments	5s
test / py314-test-mpldev-pytestdev (ubuntu-latest)	3m 30s
test / py310-test-mpl35-pytest62 (ubuntu-latest)	2m 48s
test / py314-test-mpl310 (ubuntu-latest)	3m 21s
test / py314-test-mpl310 (windows-latest)	3m 32s
test / py313-test-mpl310 (ubuntu-latest)	3m 11s
test / py314-test-mpl310 (macos-latest)	2m 54s
test / py312-test-mpl39 (ubuntu-latest)	3m 3s
test / py312-test-mpl39 (macos-latest)	3m 44s
test / py313-test-mpl39 (ubuntu-latest)	3m 12s
test / py312-test-mpl38-pytest74 (ubuntu-latest)	2m 52s
test / py310-test-mpl35 (windows-latest)	3m 5s
test / py310-test-mpl35 (macos-latest)	2m 40s
test / py313-test-mpl38-pytest84 (ubuntu-latest)	4m 55s
test / py312-test-mpl39 (windows-latest)	3m 42s
test / py310-test-mpl35 (ubuntu-latest)	3m 2s
publish	0s
	49m 36s

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