Update test_requirements.txt

`meld3` has reached 1.0.0.

test_requirements.txt

@@ -1,5 +1,3 @@
 # Requirements used for testing
 mock >= 0.5.0
-
-# TODO change this to "meld3 >= 1.0.0" once it is released
-https://github.com/Supervisor/meld3/tarball/master
+meld3 >= 1.0.0