ci: pin ubuntu versions #675

Job	Run time
Test with python 3.6 on windows-latest	1m 44s
Test with python 3.7 on windows-latest	1m 19s
Test with python 3.8 on windows-latest	10m 46s
Test with python 3.12 on windows-latest	10m 30s
Test with python 3.8 on macos-latest	3m 24s
Test with python 3.12 on macos-latest	3m 19s
Test with python 2.7 on ubuntu-20.04	39s
Test with python 3.6 on ubuntu-20.04	38s
Test with python 3.7 on ubuntu-22.04	34s
Test with python 3.8 on ubuntu-24.04	4m 5s
Test with python 3.9 on ubuntu-24.04	4m 5s
Test with python 3.10 on ubuntu-24.04	4m 17s
Test with python 3.11 on ubuntu-24.04	4m 8s
Test with python 3.12 on ubuntu-24.04	4m 55s
	54m 23s

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