Tests

explicit and use-site variance #1926

Job	Run time
Test mypyc suite with 32-bit Python	7m 32s
Test suite with py38-ubuntu, mypyc-compiled	25m 47s
Test suite with py38-windows-64	34m 44s
Test suite with py39-ubuntu	34m 17s
Test suite with py310-ubuntu	30m 53s
Test suite with py311-ubuntu, mypyc-compiled	22m 19s
Test suite with py312-ubuntu, mypyc-compiled	24m 50s
Test suite with py313-ubuntu, mypyc-compiled	24m 52s
mypyc runtime tests with py39-macos	6m 7s
Type check our own code (py38-ubuntu)	3m 50s
Type check our own code (py38-windows-64)	4m 0s
Formatting and code style with Black + ruff	56s
	3h 40m 7s

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