Add python 3.8 to travis and maybe remove python 3.5

[chaithyagr]

Currently, the travis tests are done in python 3.5, 3.6 and 3.7

However, scikit-learn has progressed to work on python 3.6 and above.
With this we have issues with travis testing in python 3.5:

 RuntimeError: Scikit-learn requires Python 3.6 or later. The current Python version is 3.5.6 installed in /home/travis/miniconda/envs/testenv/bin/python.
2065

[chaithyagr]

Adding to coding sprint.

[chaithyagr]

We have added python 3.8 already. So all we need to decide is if we should remove python 3.5?
In my opinion this reduces the amount of testing needed.

[philouc]

Yes please remove python3.5

[chaithyagr]

This is done..