Fix enum attributes are not members #13767

Job	Run time
Test mypyc suite with 32-bit Python	7m 5s
Test suite with py38-ubuntu, mypyc-compiled	17m 47s
Test suite with py38-windows-64	20m 3s
Test suite with py39-ubuntu	16m 55s
Test suite with py310-ubuntu	15m 44s
Test suite with py311-ubuntu, mypyc-compiled	15m 39s
Test suite with py312-ubuntu, mypyc-compiled	18m 9s
Test suite with py313-ubuntu, mypyc-compiled	18m 43s
mypyc runtime tests with py39-macos	5m 40s
Type check our own code (py38-ubuntu)	1m 58s
Type check our own code (py38-windows-64)	2m 12s
Formatting and code style with Black + ruff	34s
	2h 20m 29s

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