`pytest-runner` describes itself as deprecated in its own package
description, referring to
https://github.com/pypa/setuptools/issues/1684.  Using it in
`setup_requires` means that any other package that depends on Paste has
to install `pytest-runner` too, which is fairly heavyweight and
unnecessary.  (I ran into this when trying to update Launchpad to a less
ancient version of Paste.)

This does mean that `python setup.py test` no longer works, but,
according to the setuptools issue above, that's deprecated anyway.
Paste already has `tox` configuration that works well.

* 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/

This is for the sake of getting something out there, including
fixes to get stuff working with Python 3.7.

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