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>

Simplified mechanized release process through tox

fix support for easy_install's find-links option in setup.cfg

Merge pull request #1922 from benoit-pierre/fix_possible_issue_with_transitive_build_dependencies_from_extras

fix possible issue with transitive build dependencies