Travis CI: Upgrade to Python 3.8 (#1783)

* Travis CI: Upgrade to Python 3.8

* updating DIRECTORY.md

* Tensorflow is not yet compatible with Python 3.8

* Disable k_means_clustering_tensorflow.py

* updating DIRECTORY.md

* Disable gan.py

* updating DIRECTORY.md

* Disable input_data.py

* updating DIRECTORY.md

* pip install a current version of six