Defer finalize options

Avoid loading working set during `Distribution.finalize_options` prior to invoking `_install_setup_requires`, broken since v42.0.0. Fixes #2158.

honor order attribute in "finalize_distibution_options" group of entrypoints

Pin virtualenv to <20 as workaround for #1998.

fixes #1993

Bootstrap the environment in tox, allowing simple 'tox' to run tests and simplifying all of the pipelines.

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 for Python 4: replace unsafe six.PY3 with PY2

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>