Add docs requirements file, update tox & Travis config to fix RtD and builds

[brainwane]

For http://twine.readthedocs.io/ to build, the Read the Docs configuration needs a requirements.txt file in docs/.

I tested this at https://my-fork-of-twine.readthedocs.io/ and verified that this requirements.txt file makes it work. For reference, other configuration bits I had to twiddle in the Admin: Advanced Settings page on RtD:

• check the "Install your project inside a virtualenv using setup.py install" box
• point to docs/requirements.txt in the "Requirements file" box
• Python interpreter: CPython 3.x

[codecov]

Codecov Report

Merging #297 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #297   +/-   ##
=======================================
  Coverage   67.42%   67.42%           
=======================================
  Files          12       12           
  Lines         574      574           
  Branches       91       91           
=======================================
  Hits          387      387           
  Misses        161      161           
  Partials       26       26

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[di]

Might be a good time to update the 'Building the documentation' section of the docs to direct the user to just pip install -r docs/requirements.txt instead of each individual requirement.

[brainwane]

@di Thanks for the suggestion. Done.

[sigmavirus24]

I missed this in the initial review. You don't need to install those to run tox -e docs.

Instead, let's remove this and rewrite this to just be

Then, build the docs using ``tox``:

And we can update tox.ini to install from our requirements file so everything works the same way.

[brainwane]

Thanks for the suggestion, @sigmavirus24. I looked at the tox documentation and believe I've now successfully done this. f298a8e builds fine on my machine in a virtualenv that does not have Sphinx, et alia installed, and the fresh build of my test project on RtD built successfully. So, ready for re-review.

[brainwane]

Evidently PEP8 wants to use Python 2.7 or else it complains: twine/utils.py:39:18: F821 undefined name 'raw_input'.

[sigmavirus24]

I guess we need to rewrite https://github.com/pypa/twine/blob/master/twine/utils.py#L39 to look like

try:
    input_func = raw_input
except NameError:  # Python 3
    input_func = input

But that's for a different PR.