Encrypt Zulip webhook address (GH-10010)

This should reduce false failure reports to the Zulip 'core/test runs'
stream from Travis failures on private forks.

.travis.yml

@@ -165,11 +165,6 @@ script:
     fi
 notifications:
   email: false
-  webhooks:
-    urls:
-      - https://python.zulipchat.com/api/v1/external/travis?api_key=QTP4LAknlFml0NuPQmAetvH4KQaokiQE&stream=core%2Ftest+runs
-    on_success: change
-    on_failure: always
   irc:
     channels:
       # This is set to a secure variable to prevent forks from notifying the
@@ -180,3 +175,11 @@ notifications:
     on_success: change
     on_failure: always
     skip_join: true
+  webhooks:
+    urls:
+      # For the same reasons as above for IRC, we encrypt the webhook address
+      # for Zulip.  The actual value is:
+      # https://python.zulipchat.com/api/v1/external/travis?api_key=<api-key-redacted>&stream=core%2Ftest+runs
+      - secure: "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"
+    on_success: change
+    on_failure: always