Bump version to 1.12.0+dev #12785

Job	Run time
Test mypyc suite with 32-bit Python	7m 4s
Test suite with py38-ubuntu, mypyc-compiled	16m 59s
Test suite with py38-windows-64	20m 1s
Test suite with py39-ubuntu	16m 54s
Test suite with py310-ubuntu	15m 32s
Test suite with py311-ubuntu, mypyc-compiled	15m 50s
Test suite with py312-ubuntu, mypyc-compiled	17m 36s
mypyc runtime tests with py39-macos	4m 17s
mypyc runtime tests with py38-debug-build-ubuntu	4m 36s
Type check our own code (py38-ubuntu)	1m 42s
Type check our own code (py38-windows-64)	2m 21s
Formatting and code style with Black + ruff	34s
	2h 3m 26s

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