Python 3.6. CI fails in PR #13

[sonjaleo]

For generating t-SNE plots, scikit-learn Version 1.0.1 is used.
As the requirement for scikit-learn Version 1.0.1 Python 3.7 or higher is needed (https://scikit-learn.org/stable/install.html), therefore the CI for Python 3.6. fails.
Therefore, I will deactivate the CI checks for Python 3.6. so tests will only run on Python 3.8.

PR: #13