Merge pull request #7132 from tk0miya/stop_testing_with_nightly

Stop to test with nightly python

Author: tk0miya

.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'