chore: test minimum dependencies in python 3.7 (#397)

* chore: test minimum dependencies in python 3.7

* remove duplicate entry

testing/constraints-3.7.txt

@@ -0,0 +1,15 @@
+# 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 "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
+googleapis-common-protos==1.56.2
+protobuf==3.15.0
+google-auth==1.25.0
+requests==2.18.0
+packaging==14.3
+grpcio==1.33.2
+grpcio-gcp==0.2.2
+grpcio-status==1.33.2