pip 10 support

[mikebohdan]

Support of pip 10 and later. Details pypa/pip#5272

[coveralls]

Coverage remained the same at 100.0% when pulling 93d3051 on mikebohdan:feature/pip-10-support into 9a75742 on steenzout:master.

[mikebohdan]

@steenzout, can I remove py3.3 support? Even setup_tools doesn't support it

Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.3.6/bin/tox", line 7, in <module>
    from tox import cmdline
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/tox/__init__.py", line 32, in <module>
    from .session import cmdline  # noqa
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/tox/session.py", line 18, in <module>
    import pkg_resources
  File "/home/travis/virtualenv/python3.3.6/lib/python3.3/site-packages/pkg_resources/__init__.py", line 90, in <module>
    raise RuntimeError("Python 3.4 or later is required")
RuntimeError: Python 3.4 or later is required
The command "tox" exited with 1.

[steenzout]

sure.