Fix special cases of kwargs + TypeVarTuple #13114

Job	Run time
Test mypyc suite with 32-bit Python	6m 47s
Test suite with Python nightly	14m 0s
Test suite with py38-ubuntu, mypyc-compiled	18m 19s
Test suite with py38-windows-64	19m 48s
Test suite with py39-ubuntu	16m 41s
Test suite with py310-ubuntu	15m 17s
Test suite with py311-ubuntu, mypyc-compiled	15m 50s
Test suite with py312-ubuntu, mypyc-compiled	17m 31s
mypyc runtime tests with py39-macos	4m 44s
mypyc runtime tests with py38-debug-build-ubuntu	4m 26s
Type check our own code (py38-ubuntu)	1m 51s
Type check our own code (py38-windows-64)	2m 20s
Formatting and code style with Black + ruff	38s
	2h 18m 12s

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