Instead of installing setup_requires, which escapes buildout versions
constraints and allow-picked-versions, we disallow setup_requires to
install any missing packages. The packages are supposed to have been
installed earlier, by using setup-eggs= option of zc.recipe.egg:custom
or other variants of this recipe.

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

Rename 'Professional support' to 'For Enterprise' and add section on 'For Enterprise' to the README (linking to Tidelift).

Handle the case where a missing transitive build dependency is
required by an extra for an already installed build dependency.