chore: Pin depencies to lower bounds in constraints for lowest Python version tests

Author: vchudnov-g

testing/constraints-3.9.txt

@@ -0,0 +1,14 @@
+# This constraints file is used to check that lower bounds
+# are correct in pyproject.toml
+# List *all* library dependencies and extras in this file.
+# Pin the version to the lower bound.
+#
+# e.g., if pyproject.toml has "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
+googleapis-common-protos==1.56.2
+protobuf==4.25.8
+google-auth==2.14.1
+requests==2.18.0
+grpcio==1.33.2
+grpcio-status==1.33.2
+proto-plus==1.22.3