Travis: run mypy with Python 3.8 as well (#3146)

This runs mypy both with Python 3.7 and 3.8. In Python 3.8,
mypy switched from using typed-ast to using Python's built-in ast.
This patch ensures that both are tested.

tests/mypy_test.py

@@ -88,7 +88,7 @@ def main():
         print("Cannot import mypy. Did you install it?")
         sys.exit(1)
 
-    versions = [(3, 7), (3, 6), (3, 5), (3, 4), (2, 7)]
+    versions = [(3, 8), (3, 7), (3, 6), (3, 5), (3, 4), (2, 7)]
     if args.python_version:
         versions = [v for v in versions
                     if any(('%d.%d' % v).startswith(av) for av in args.python_version)]