add a comment explaining why we're excluding pypy3 on linux
diff --git a/.travis.yml b/.travis.yml
index 3800868..9b900eb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -43,6 +43,7 @@
 
 matrix:
     exclude:
+        # excluding pypy3 from linux configs until the ubuntu ppa has pypy3 available.
         - os: linux
           env: TOXENV=pypy3
         - os: linux