Add __name__ field to MethodType (#1442)

See https://github.com/python/cpython/blob/08c16016e2a2d1368d001ddebfe9ca92465773c4/Lib/types.py#L32-L34

```python
>>> class _C:
...     def _m(self): pass
...
>>> _C()._m.__name__
'_m'
```

Author: rowillia

stdlib/2/types.pyi

@@ -82,6 +82,7 @@ class UnboundMethodType:
     im_class = ...  # type: type
     im_func = ...  # type: FunctionType
     im_self = ...  # type: Optional[object]
+    __name__ = ...  # type: str
     __func__ = im_func
     __self__ = im_self
     def __call__(self, *args: Any, **kwargs: Any) -> Any: ...

stdlib/3/types.pyi

@@ -139,6 +139,7 @@ class _StaticFunctionType:
 class MethodType:
     __func__ = ...  # type: _StaticFunctionType
     __self__ = ...  # type: object
+    __name__ = ...  # type: str
     def __call__(self, *args: Any, **kwargs: Any) -> Any: ...
 class BuiltinFunctionType:
     __self__ = ...  # type: Union[object, ModuleType]