CI: re-add Codecov upload #3032

Job	Run time
run-if-changes	6s
get-versions	11s
select-tests	14s
build (3.10, 3.9) / Compare baseline Python 3.10 with MPL 3.9	5m 57s
build (3.14, 3.10) / Test Python 3.14 with MPL 3.10	22s
build (3.10, 3.9) / Test Python 3.10 with MPL 3.9	26s
build (3.12, 3.10) / Test Python 3.12 with MPL 3.10	26s
build (3.14, 3.10) / Compare baseline Python 3.14 with MPL 3.10	6m 21s
build (3.12, 3.10) / Compare baseline Python 3.12 with MPL 3.10	5m 23s
build-success	3s
	19m 29s

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