memoryview is a context manager (#2442)

Since Python 3.2, __enter__ returns self and __exit__ calls self.release()

stdlib/3/builtins.pyi

@@ -495,6 +495,8 @@ class memoryview(Sized, Container[int]):
     ndim = ...  # type: int
 
     def __init__(self, obj: Union[bytes, bytearray, memoryview]) -> None: ...
+    def __enter__(self) -> memoryview: ...
+    def __exit__(self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[TracebackType]) -> bool: ...
 
     @overload
     def __getitem__(self, i: int) -> int: ...