Upgrade Sphinx version for Read the Docs

[brainwane]

What's the problem this feature will solve?
The pip documentation says in the footer: "Created using Sphinx 1.7.9." The Sphinx website says the latest version of Sphinx is 2.0.1.

Describe the solution you'd like
Update the dependency at tools/docs-requirements.txt so it pins to Sphinx 2.0.1, not sphinx == 1.7.*.

I believe one of the maintainers of the pip project on Read the Docs might also need to log in and fiddle with the preferred version of Sphinx to get things to work right.

[ppiyakk2]

I will working on it!

[ppiyakk2]

The sphinx version 2.0.1 doesn't accept python 2.7. It support over python 3.5.

This is the result of command pip install sphinx==2.0.1 -v

The package https://files.pythonhosted.org/packages/81/e3/cb726b65039569840547895e1c81759d44e50512d038268afa8c1bf2001c/Sphinx-2.0.1.tar.gz#sha256=423280646fb37944dd3c85c58fb92a20d745793a9f6c511f59da82fa97cd404b (from https://pypi.org/simple/sphinx/) (requires-python:>=3.5) is incompatible with the pythonversion in use. Acceptable python versions are:>=3.5
  Could not find a version that satisfies the requirement Sphinx==2.0.1 (from versions: 0.1.61611, 0.1.61798, 0.1.61843, 0.1.61945, 0.1.61950, 0.2, 0.3, 0.4, 0.4.1, 0.4.2, 0.4.3, 0.5, 0.5.1, 0.5.2b1, 0.5.2, 0.6b1, 0.6, 0.6.1, 0.6.2, 0.6.3, 0.6.4, 0.6.5, 0.6.6, 0.6.7, 1.0b1, 1.0b2, 1.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4, 1.0.5, 1.0.6, 1.0.7, 1.0.8, 1.1, 1.1.1, 1.1.2, 1.1.3, 1.2b1, 1.2b2, 1.2b3, 1.2, 1.2.1, 1.2.2, 1.2.3, 1.3b1, 1.3b2, 1.3b3, 1.3, 1.3.1, 1.3.2, 1.3.3, 1.3.4, 1.3.5, 1.3.6, 1.4a1, 1.4b1, 1.4, 1.4.1, 1.4.2, 1.4.3, 1.4.4, 1.4.5, 1.4.6, 1.4.7, 1.4.8, 1.4.9, 1.5a1, 1.5a2, 1.5b1, 1.5, 1.5.1, 1.5.2, 1.5.3, 1.5.4, 1.5.5, 1.5.6, 1.6b1, 1.6b2, 1.6b3, 1.6.1, 1.6.2, 1.6.3, 1.6.4, 1.6.5, 1.6.6, 1.6.7, 1.7.0b1, 1.7.0b2, 1.7.0, 1.7.1, 1.7.2, 1.7.3, 1.7.4, 1.7.5, 1.7.6, 1.7.7, 1.7.8, 1.7.9, 1.8.0b1, 1.8.0, 1.8.1, 1.8.2, 1.8.3, 1.8.4, 1.8.5)