bump python 3.7 -> 3.8 on CI #1549

Job	Run time
Wheel on macos-latest py-3.8	3m 18s
Run fuzz targets (macos-latest, 3.8)	36m 25s
Wheel on ubuntu-latest py-3.8	2m 24s
Unit tests (macos-latest, 3.8)	9m 40s
Run fuzz targets (ubuntu-latest, 3.8)	19m 46s
Wheel on windows-latest py-3.8	2m 16s
Unit tests (ubuntu-latest, 3.8)	3m 15s
Unit tests (windows-latest, 3.8)	5m 21s
Checks	46s
coverage	4m 34s
Upload to PyPI	0s
	1h 27m 45s

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