Use Python 3.9 release in Travis configuration
diff --git a/.travis.yml b/.travis.yml
index e6f04d9..72d6da7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,7 +5,7 @@
   - "3.6"
   - "3.7"
   - "3.8"
-  - "3.9-dev"
+  - "3.9"
   - "pypy3"
 install:
   - pip install .