travis: Add Python 3.8 test. (#393)

Author: Djailla

.travis.yml

@@ -4,13 +4,13 @@ language: python
 # See aws s3 ls s3://travis-python-archives/binaries/ubuntu/16.04/x86_64/
 python:
   - "nightly"
-  - "pypy3.5"
-  - "pypy2.7-5.10.0"
+  - "pypy3"
+  - "pypy"
+  - "3.8"
   - "3.7"
   - "3.6"
   - "3.5"
   - "2.7"
-  - "3.8-dev"
 
 cache: pip