Bootstrap the environment in tox, allowing simple 'tox' to run tests.

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

Include PKG-INFO in minimal egg-info so that metadata doesn't need to be generated twice during bootstrap.

Ensure bdist_wheel no longer creates a universal wheel [remove bdist_wheel block], close #1976

Add 'Programming Language :: Python :: 3 :: Only' trove classifier

by removing the [bdist_wheel] section

The project has been Python 3 only since
a46a6bfd.

Improve setuptools.txt

Update Python 2 warning to include a minimum sunset date and add a preamble to make referencing the warning more reliable. Ref #1458.

Integrate GitHub Actions CI/CD  tests

This change is necessary because macOS 10.15 is now deprecated.

Ref: https://github.blog/changelog/2019-10-31-github-actions-macos-virtual-environment-is-updating-to-catalina-and-dropping-mojave-supportCo-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>

Co-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>

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.