Remove Python 2.7 support

[stdevel]

As Python 2.7.x is EOL and the most Linux distributions and many projects already dropped support, we should do the same and use Python 3.6.

[stdevel]

Some tasks that need to be done:

• [ ] remove future imports
• [ ] remove raw_input calls
• [ ] reformat logging (remove format() calls)
• [ ] replace next() iterations

[okin]

We should also update the classifiers in setup.py if we drop support for Python 2.

[stdevel]

This is/was implemented in 43-feat-uyuni_support.