[Chapel Merge] Update to require Python 3

Branch: refs/tags/0.0.15
Revision: 0c7c241
Author: lydia-duncan
Log Message:

Merge pull request #38 from lydia-duncan/updateForNewerSphinx

Update to require Python 3
[reviewed by @mppf]

We want to drop support for Python 2 and start taking advantage of later features
of Sphinx, or at the very least not be unreasonably far behind on Sphinx.

Update our Sphinx version from 1.8.5 to 3.2.1. We’ve been on 1.8.5 for a while,
in part due to not being ready to switch to Python 3 full time. Now that we want
to start relying on Python 3, update the version of Sphinx we use.

As part of this change, replace uses of sphinx.locale.l_ with sphinx.locale._.
l_ was deprecated and ultimately removed. The deprecation message says to use
_ instead, so do that.

Updates Travis to use 3.6 to test instead of 2.7 and 3.4. Also updates setup.py and
the release script to use Python 3 as well, and for the new updates to the installation
requirements. Everything should be tested except for the release script, which I was
worried would prematurely tag the release.

This change additionally migrates to using python3 -m pip instead of pip to
install packages, since pip is included in later python releases. These changes were
originally performed by Michael on the main Chapel repo copy of this repository.

Tested locally

Modified Files:
M .travis.yml
M docs/developers.rst
M docs/index.rst
M requirements.txt
M setup.py
M sphinxcontrib/chapeldomain/init.py
M util/release.bash

Compare: https://github.com/chapel-lang/sphinxcontrib-chapeldomain/compare/0.0.15