chore: add constraints file (#45)

Add constraints file to test lower bounds. These files will not be used until the noxfile is changed in googleapis/synthtool#869.

Author: busunkim96

packages/google-cloud-os-login/testing/constraints-3.10.txt

@@ -0,0 +1,10 @@
+# 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==1.22.2
+proto-plus==1.4.0
+libcst==0.2.5