Stop testing Python 3.3 in Travis

Use canonicalize_name to look for .dist-info in wheel files

Add tests for PEP 425 support

Fix attr: with package_dirs

Generate doc previews for pull requests

Fixes issue #1350

Support loading version from a file

Pull request template

Add changelog for PR #1363 (issue #1355)

Fix doc build warnings

Add configuration for towncrier

Small fixes to Docs:Developer Guide

Add tox environment for docs

Add coverage badge to README

This shows the coverage of the master branch as measured by Codecov.

Pin pytest-flake8 to 1.0.0

pytest-flake8 1.0.1 has a hard dependency on pytest>=3.5 [1], which
does not support python3.3 [2].  Avoid it for python3.3 and below
until python3.3 is removed [3].

Additionally, ensure we have a recent enough pip in travis ci to
handle multiple requirements entries.

[1] https://github.com/tholo/pytest-flake8/commit/25bbd3b42d3aa0962fb736202115dae9e5d2cd7c
[2] https://docs.pytest.org/en/latest/changelog.html#pytest-3-3-0-2017-11-23
[3] https://github.com/pypa/setuptools/pull/1342

Update all pypi.python.org URLs to pypi.org