Restore testing on Python 2, bypassing the requires-python check when installing Setuptools.

Rely on tox-pip-version to upgrade pip

Rely on tox-pip-version to upgrade pip and minimize the hack for removing setuptools from the environment.

Fix dead link

Fix TestPyPI upload URI

Require Python 3.5 or later, dropping support for Python 2.

Fix for Python 4: replace unsafe six.PY3 with PY2

Add obnoxious warning about Python 2 being unsupported on this release with guidance on how to avoid the warning and what to do if that guidance was ineffective.

This change does not yet remove any of the compatibility for Python 2, but only aims to declare the dropped support.

drop support for Python 3.4

Update setuptools.txt

Include pyproject.toml in sdist

Restore build-backend and remove switch to avoid pep517.

Co-Authored-By: Paul Ganssle <paul@ganssle.io>