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

This is both compatible with the old version of wheel (last one
supported under Python 3.3) and is more in line with our commitment,
which is that the wheel install provides at least these files.

Set up code coverage

Rationale:
- tox should work the same as before
- coverage is too slow and disabled by default locally
- disable coverage on pypy and pypy3 (way too slow)
- add coverage info for pkg_resources
- make sure tests are ignored in coverage
- make sure all CI jobs are identifiable
- include coverage for xfail (too complicated otherwise)
- disable report on terminal

tests: Add regression test for #1081

This ensures we don't unknowingly change the behavior of this again.
Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: #1307

On pypy3 on Travis (version 5.8.0), running with coverage
enabled causes significant slowdown and test_build_meta
ends up timing out or otherwise failing. Until that is
fixed, skip coverage on pypy3 on Travis.

Some of these environment variables are necessary codecov to
detect that it is on travis/appveyor and thus doesn't need
an upload token.

Stop patching write_pkg_info

Fix unicode maintainer