Some tools, such as PBR, might want to set
long_description_content_type during the parent object's
Distribution.__init__() call (during distutils setup_keywords entry
points).  However, that field is currently unconditionally overwritten
after these calls, erasing the value.

We would rather not duplicate the existing method of copying into
dist.metadata as done with project_urls.

This preserves the fields within Distribution.metadata described by
self._DISTUTIULS_UNUPPORTED_METADATA, or otherwise takes it from
arguments.

A test case that simulates setting the long description and overriding
the arguments is added.

Generate doc previews for pull requests

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

For details on the new PyPI, see the blog post:

https://pythoninsider.blogspot.ca/2018/04/new-pypi-launched-legacy-pypi-shutting.html

pep425tags: turn warning into a debug log entry

fixes #1314

Add .pytest_cache/ to gitignore

Update wheel tests to reflect latest version