make IOBuf.__iter__ yield Python builtin memoryview

Summary:
So it may be easier to use in Python code.

A lot of Python code will check the type with this:
```
if isinstance(obj, (bytes, memoryview)):
  ...
```

Currently, the `IOBuf.__iter__` returns a `cython.view.memoryview` c-type (which can be only "cimport" in cython but not able to "import), and to make it useable in scenarios like above, we have to explicitly cast it to builtin `memoryview`.

With this change, the use of the iterator is more natural.

Reviewed By: fried

Differential Revision: D25577842

fbshipit-source-id: 6b22028d06bba639fca32f7de3a063e1fb1baa15

folly/python/iobuf.pyi

@@ -28,9 +28,7 @@ class IOBuf(Hashable):
     def __bytes__(self) -> bytes: ...
     def __bool__(self) -> bool: ...
     def __len__(self) -> int: ...
-    def __iter__(
-        self
-    ) -> Iterator[bytes]: ...  # Lies, but typing likes it for b''.join()
+    def __iter__(self) -> Iterator[memoryview]: ...
     def __hash__(self) -> int: ...
     def __eq__(self, other: Any) -> bool: ...
     def __ne__(self, other: Any) -> bool: ...

folly/python/iobuf.pyx

@@ -12,12 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from builtins import memoryview as py_memoryview
 from folly.executor cimport get_executor
 from cpython cimport Py_buffer
 from weakref import WeakValueDictionary
 from cpython.object cimport Py_LT, Py_LE, Py_EQ, Py_NE, Py_GT, Py_GE
 from cython.operator cimport dereference as deref
-from cython.view cimport memoryview
 
 __cache = WeakValueDictionary()
 __all__ = ['IOBuf']
@@ -135,10 +135,10 @@ cdef class IOBuf:
 
     def __iter__(self):
         "Iterates through the chain of buffers returning a memory view for each"
-        yield memoryview(self, PyBUF_C_CONTIGUOUS)
+        yield py_memoryview(self)
         next = self.next
         while next is not None and next is not self:
-            yield memoryview(next, PyBUF_C_CONTIGUOUS)
+            yield py_memoryview(next)
             next = next.next
 
     def __hash__(self):