Improve tests setup in GH Actions CI/CD workflow

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

As per #1961.

P.S. Using `contains(['2.7', 'pypy2'], matrix.python-version)` does not
work even though
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/contexts-and-expression-syntax-for-github-actions#contains
promises arrays to be supported.

Before this change, a deprecated `version` arg was used.

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