Upgrade mypy & DataWithCoords check (#3044)

Author: max-sixty

ci/requirements-py36.yml

@@ -31,4 +31,4 @@ dependencies:
   - pydap
   - lxml
   - pip:
-    - mypy==0.660
+    - mypy==0.711

xarray/core/common.py

@@ -996,6 +996,10 @@ def __enter__(self: T) -> T:
     def __exit__(self, exc_type, exc_value, traceback) -> None:
         self.close()
 
+    def __getitem__(self, value):
+        # implementations of this class should implement this method
+        raise NotImplementedError
+
 
 def full_like(other, fill_value, dtype: Optional[DTypeLike] = None):
     """Return a new object with the same shape and type as a given object.