[mypyc] feat: `__mypyc_empty_tuple__` constant #17712

Job	Run time
Test mypyc suite with 32-bit Python	10m 11s
mypyc runtime tests with py39-macos	6m 37s
Test suite with py312-ubuntu, mypyc-compiled	19m 39s
Test suite with py310-ubuntu	10m 17s
Test suite with py313-windows-64	22m 27s
Test suite with py311-ubuntu	9m 38s
Test suite with py314-dev-ubuntu	25m 37s
Test suite with py39-ubuntu, mypyc-compiled	16m 6s
Formatting and code style with Black + ruff	2m 11s
Test suite with py313-ubuntu, mypyc-compiled	20m 4s
Type check our own code (py39-ubuntu)	1m 43s
Type check our own code (py39-windows-64)	2m 10s
	2h 26m 40s

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