TEST: Add foundation for TypeVar defaults (PEP 696) #265

Job	Run time
Test mypyc suite with 32-bit Python	8m 19s
Test suite with py38-ubuntu, mypyc-compiled	19m 59s
Test suite with py38-windows-64	26m 13s
Test suite with py39-ubuntu	18m 36s
Test suite with py310-ubuntu	17m 37s
Test suite with py311-ubuntu, mypyc-compiled	17m 28s
Test suite with py312-ubuntu, mypyc-compiled	20m 16s
mypyc runtime tests with py39-macos	4m 55s
mypyc runtime tests with py38-debug-build-ubuntu	5m 24s
Type check our own code (py38-ubuntu)	1m 47s
Type check our own code (py38-windows-64)	2m 31s
Formatting and code style with Black + ruff	38s
	2h 23m 43s

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