Don't require the Python org to mention someone (GH-393)

Previously we configured the mention-bot to only mention people who are members of the Python organization. However, this doesn't currently work
if members don't have their membership public. Instead we will configure mention-bot to poke anyone, even non-members.

.mention-bot

@@ -7,6 +7,5 @@
     "Misc/NEWS"
   ],
   "userBlacklist": ["gvanrossum"],
-  "userBlacklistForPR": ["benjaminp"],
-  "requiredOrgs": ["python"]
+  "userBlacklistForPR": ["benjaminp"]
 }