Upgrade python/apitools to v0.5.31

Exempt-From-Owner-Approval:
Change-Id: I87e804ff19a93d51d6244f834bceebc32536801e
diff --git a/METADATA b/METADATA
index 536369d..e5ecb93 100644
--- a/METADATA
+++ b/METADATA
@@ -9,11 +9,11 @@
     type: GIT
     value: "https://github.com/google/apitools"
   }
-  version: "v0.5.30"
+  version: "v0.5.31"
   license_type: NOTICE
   last_upgrade_date {
-    year: 2019
-    month: 6
-    day: 26
+    year: 2020
+    month: 5
+    day: 14
   }
 }