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'
```

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]