diff --git a/setup.py b/setup.py
index f1bcfb0..8a8c989 100644
--- a/setup.py
+++ b/setup.py
@@ -28,6 +28,7 @@
     "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
     "libcst >= 0.2.5",
     "proto-plus >= 1.15.0",
+    "packaging >= 14.3",
 ]
 
 package_root = os.path.abspath(os.path.dirname(__file__))
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 1836bd9..0b5003a 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -22,3 +22,5 @@
 google-api-core==1.22.2
 libcst==0.2.5
 proto-plus==1.15.0
+packaging==14.3
+google-auth==1.24.0  # TODO: remove when google-auth>=1.25.0 si transitively required through google-api-core