Drop python2.7 support

[camporter]

Due to python 2.7 losing official support in January 2020, consider an update that drops support for it.

• Remove six.
• Remove unnecessary __future__ imports.
• Remove py27 from tox and travisci build.
• Replace pypy with pypy3.
• Add python_requires to setup.py and set the minimum to python 3.5.

Other things to consider:

• Fail with a message if library or CLI still used on python 2.

This should probably be considered for a new major version.

[coveralls]

Coverage decreased (-0.005%) to 93.682% when pulling 78f196f on camporter:drop_python2 into 880f23e on softlayer:master.

[allmightyspiff]

Thanks for removing all the old py2 stuff :)

Going to just test a bit locally before merging, but the changes themselves look fine to me.

Coverage decreased (-0.005%) is also fine btw.