Opt into PEP 517 for pip installs to avoid implicit dependency on Setuptools to install test dependencies.

Move test dependencies into the package, removing 'tests' directory which masks the error reported in #1896.

Rewrite selective PIP_IGNORE_REQUIRES_PYTHON to allow test dependencies in metadata to be installed without ignoring REQUIRES_PYTHON

Fix _imp module behaviour if is defined paths in find_spec call

Separate test for 'is_present' into its own test and add a TODO about this behavior being apparently unused.

Add flake8-2020 to tests/requirements.txt

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.