chore: test minimum dependencies in python 3.7 (#321)

Author: parthea

packages/proto-plus/testing/constraints-3.7.txt

@@ -0,0 +1,9 @@
+# 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
+google-api-core==1.31.5
+protobuf==3.19.0