Remove use of six

[joshuagl]

Description of issue or feature request:

Following the release of tuf 0.17.0 (see #1284) we are no longer supporting Python2, and have started to remove uses of Python2 in #1293 (see also #1276).

Six is a Python 2 and 3 compatibility library, which is clearly no longer required.

Current behavior:

Six is marked as a dependency in setup.py and our requirements files.
Six is imported and used in multiple modules, tests, and scripts.

Expected behavior:

six is no longer a dependency and is not imported, or used, in any modules, tests, or scripts.

[avelichka]

Hi folks,

I'd like to work on this issue if it's OK :)