fix(deps): require six>=1.13.0 (#1030)

This is needed for collections_abc

Author: busunkim96

setup.py

@@ -42,7 +42,7 @@
     "google-auth>=1.16.0",
     "google-auth-httplib2>=0.0.3",
     "google-api-core>=1.21.0,<2dev",
-    "six>=1.6.1,<2dev",
+    "six>=1.13.0,<2dev",
     "uritemplate>=3.0.0,<4dev",
 ]