Add CI testing against 3.12 #569

Job	Run time
Unit Tests (ubuntu-latest, 3.8)	2m 30s
Unit Tests (ubuntu-latest, 3.9)	2m 29s
Unit Tests (ubuntu-latest, 3.10)	2m 39s
Unit Tests (ubuntu-latest, 3.11)	2m 28s
Unit Tests (ubuntu-latest, 3.12)	3m 14s
Unit Tests (windows-latest, 3.8)	3m 41s
Unit Tests (windows-latest, 3.9)	4m 50s
Unit Tests (windows-latest, 3.10)	3m 45s
Unit Tests (windows-latest, 3.11)	4m 45s
Unit Tests (windows-latest, 3.12)	5m 20s
Unit Tests (macos-latest, 3.8)	2m 55s
Unit Tests (macos-latest, 3.9)	3m 45s
Unit Tests (macos-latest, 3.10)	3m 5s
Unit Tests (macos-latest, 3.11)	3m 27s
Unit Tests (macos-latest, 3.12)	3m 42s
Indicate completion to coveralls.io	14s
	52m 49s

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