Use TypeVar defaults for `Generator` and `AsyncGenerator` #23981

Job	Run time
Run mypy against the stubs (linux, 3.8)	1m 29s
Run pytype against the stubs	1m 32s
Run mypy against the stubs (linux, 3.9)	1m 43s
Test typeshed with pyright (Linux, 3.8)	54s
Run mypy against the stubs (linux, 3.10)	1m 40s
Test typeshed with pyright (Linux, 3.9)	55s
Run mypy against the stubs (linux, 3.11)	1m 38s
Run mypy on the test cases	3m 20s
Test typeshed with pyright (Linux, 3.10)	56s
Run mypy against the stubs (linux, 3.12)	1m 49s
Test typeshed with pyright (Linux, 3.11)	56s
Run mypy against the stubs (win32, 3.8)	1m 34s
Test typeshed with pyright (Linux, 3.12)	54s
Run the stub_uploader tests	3m 0s
Run mypy against the stubs (win32, 3.9)	1m 39s
Test typeshed with pyright (Windows, 3.8)	53s
Run mypy against the stubs (win32, 3.10)	1m 33s
Run mypy against the stubs (win32, 3.11)	1m 40s
Check typeshed structure	8s
Test typeshed with pyright (Windows, 3.9)	59s
Run mypy against the stubs (win32, 3.12)	1m 49s
Test typeshed with pyright (Windows, 3.10)	55s
Run mypy against the stubs (darwin, 3.8)	1m 38s
Test typeshed with pyright (Windows, 3.11)	1m 3s
Run mypy against the stubs (darwin, 3.9)	1m 37s
Run mypy against the stubs (darwin, 3.10)	1m 41s
Test typeshed with pyright (Windows, 3.12)	56s
Run mypy against the stubs (darwin, 3.11)	1m 33s
Run mypy against the stubs (darwin, 3.12)	1m 43s
Test typeshed with pyright (Darwin, 3.8)	59s
Test typeshed with pyright (Darwin, 3.9)	57s
Test typeshed with pyright (Darwin, 3.10)	55s
Test typeshed with pyright (Darwin, 3.11)	53s
Test typeshed with pyright (Darwin, 3.12)	53s
	46m 44s

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