* pytest: fix collection warnings via __test__=False

Fixes

> "cannot collect test class %r because it has a __init__ constructor

Ref: https://github.com/pytest-dev/pytest/issues/2007

* pytest: configure testpaths

This is faster with test collection.

* pytest: fix warning with doctests

Fixes

> /usr/lib/python3.7/site-packages/_pytest/python.py:764:
> RemovedInPytest4Warning: usage of Generator.Function is deprecated,
> please use pytest.Function instead

* Minor fixes around s/py.test/pytest/

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

See https://www.python.org/dev/peps/pep-0479/