fix: fix mypy errors

Author: parthea

packages/google-cloud-monitoring/google/cloud/monitoring_v3/_dataframe.py

@@ -19,7 +19,7 @@
 try:
     import pandas
 except ImportError:  # pragma: NO COVER
-    pandas = None
+    pandas = None  # type: ignore[assignment]
 
 from google.cloud import monitoring_v3
 

packages/google-cloud-monitoring/noxfile.py

@@ -101,6 +101,7 @@ def mypy(session):
         "mypy<1.16.0",
         "types-requests",
         "types-protobuf",
+        "pandas-stubs",
     )
     session.install(".")
     session.run(