Remove uses of `from __future__ import`

[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).

Various incompatible changes were made available to Python 2.7 via the __future__, where importing from the module would ensure that features worked the same across Python 2 and 3.

Now that we are no longer supporting Python 2 we do not need to import from the future module.

Current behavior:

from __future__ import pattern is used in 168 places in the tuf codebase.

Expected behavior:

from __future__ import pattern is not used in the code.