diff --git a/setup.py b/setup.py
index 7ef9d2e5..ec122328 100644
--- a/setup.py
+++ b/setup.py
@@ -28,7 +28,11 @@
 # 'Development Status :: 4 - Beta'
 # 'Development Status :: 5 - Production/Stable'
 release_status = "Development Status :: 5 - Production/Stable"
-dependencies = ["google-api-core[grpc] >= 1.22.2, < 2.0.0dev", "proto-plus >= 1.10.0"]
+dependencies = [
+    "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
+    "proto-plus >= 1.10.0",
+    "packaging >= 14.3",
+]
 extras = {"libcst": "libcst >= 0.2.5"}
 scripts = ["scripts/fixup_keywords.py"]
 
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 88b0c2fe..65c77e80 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -8,3 +8,5 @@
 google-api-core==1.22.2
 proto-plus==1.10.0
 libcst==0.2.5
+packaging==14.3
+google-auth==1.24.0  # TODO: remove when google-auth>=1.25.0 is transitively required through google-api-core