Fix enum attributes are not members #13764

Job	Run time
Test mypyc suite with 32-bit Python	7m 32s
Test suite with py38-ubuntu, mypyc-compiled	18m 3s
Test suite with py38-windows-64	20m 13s
Test suite with py39-ubuntu	16m 56s
Test suite with py310-ubuntu	16m 5s
Test suite with py311-ubuntu, mypyc-compiled	16m 9s
Test suite with py312-ubuntu, mypyc-compiled	17m 43s
Test suite with py313-ubuntu, mypyc-compiled	18m 14s
mypyc runtime tests with py39-macos	6m 43s
Type check our own code (py38-ubuntu)	1m 58s
Type check our own code (py38-windows-64)	2m 43s
Formatting and code style with Black + ruff	32s
	2h 22m 51s

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