chore: test minimum dependencies in python 3.7 (#15)

Author: parthea

packages/google-cloud-network-services/testing/constraints-3.7.txt

@@ -0,0 +1,8 @@
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List all library dependencies and extras in this file.
+# Pin the version to the lower bound.
+# e.g., if setup.py has "google-cloud-foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have google-cloud-foo==1.14.0
+google-api-core==1.31.5
+proto-plus==1.15.0