Stop to test with nightly python

It seems nightly python image on Travis CI has broken now.  This
stops to test with it temporarily to make our development keep going.
https://travis-ci.community/t/python-development-versions-no-longer-include-pip/7180

.travis.yml

@@ -24,7 +24,7 @@ matrix:
       env:
         - TOXENV=du15
         - PYTEST_ADDOPTS="--cov ./ --cov-append --cov-config setup.cfg"
-    - python: 'nightly'
+    - python: '3.8'
       env:
         - TOXENV=du16
     - python: '3.6'