MNT: drop deprecated image ubuntu-20.04 #342

Job	Run time
tests / Load tox environments	4s
tests / style (ubuntu-latest)	43s
tests / py38-test-oldestdeps (ubuntu-latest)	44s
tests / py39-test (ubuntu-latest)	57s
tests / py310-test (ubuntu-latest)	1m 1s
tests / py311-test (ubuntu-latest)	1m 2s
tests / py312-test (ubuntu-latest)	1m 25s
tests / py312-test-devdeps (ubuntu-latest)	1m 21s
tests / py312-test-devdeps (macos-latest)	1m 5s
tests / py38-test (windows-2019)	2m 45s
tests / py312-test-devdeps (windows-2019)	3m 13s
tests / py39-test-osxclang-conda (macos-12)	1s
tests / py39-test-linuxgcc-conda (ubuntu-latest)	3m 54s
tests / py312-downstream (ubuntu-latest)	8m 35s
publish	0s
	26m 50s

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