Use Generator TypeVar defaults (#18384)

`collections.abc.Generator` doesn't check the number of TypeVars (in
contrast to `typing.Generator`). So it's possible to use
`Generator[None]` even for Python 3.9.

mypyc/irbuild/match.py

@@ -332,7 +332,7 @@ def bind_as_pattern(self, value: Value, new_block: bool = False) -> None:
                 self.builder.goto(self.code_block)
 
     @contextmanager
-    def enter_subpattern(self, subject: Value) -> Generator[None, None, None]:
+    def enter_subpattern(self, subject: Value) -> Generator[None]:
         old_subject = self.subject
         self.subject = subject
         yield