pytest exposes many warnings, some but not all of which are cleaned
up here. The main switch is to use html.escape instead of cgi.escape.
This inspired the addition of 'future' to requirements.

The remaining warnings are related to pytest deprecations or over-eager
test discovery.

It is perhaps ironic that the switch to pytest is to avoid nose being
mostly dead, and now we are using features in pytest that pytest wants
to make dead. These are left for later cleanups, which means that
running the tests is noisy.

Switch from nose to pytest

The proxy test relied on pythonpaste.org being up and existing.
It is neither. So instead we use httpbin.org which is often used
for this kind of thing. Unfortunately httpbin is now a react app
which means a lot of the HTML is generated client-side, which
means we need to choose wisely.

As the original comments indicate, the test is not particularly
robust and remains so.

This involved:

- Adding "pypy" to the `tox.ini` envlist.
- Adding `# doctest: +IGNORE_EXCEPTION_DETAIL` to 2 doctests, because the
  traceback message text is slightly different on PyPy.