Fix mypy tests (#8476)

I was seeing an error in #8475

Author: max-sixty

xarray/core/nputils.py

@@ -31,7 +31,7 @@
     _HAS_NUMBAGG = Version(numbagg.__version__) >= Version("0.5.0")
 except ImportError:
     # use numpy methods instead
-    numbagg = np
+    numbagg = np  # type: ignore
     _HAS_NUMBAGG = False