fix: pin grpcio / googleapis-common-protos under Python2 (#725)

Both dropped Python2 support in a minor release.  grpcio does not declare
'python_requires'.

Author: tseaver

packages/google-cloud-ndb/setup.py

@@ -26,6 +26,9 @@ def main():
         readme = readme_file.read()
     dependencies = [
         "google-cloud-datastore >= 1.7.0, < 2.0.0dev",
+        "googleapis-common-protos < 1.53.0; python_version<'3.0'",
+        "grpcio < 1.40dev; python_version<'3.0'",
+        "protobuf < 3.18dev; python_version<'3.0'",
         "pymemcache",
         "redis",
         "pytz"