Fix install_scripts() if bdist_wininst is missing

Move test dependencies to package metadata.

Closes #1985

Move docs dependencies into package metadata and use same technique as jaraco/skeleton to build docs in tox and rtd.

Improve tests setup in GH Actions CI/CD workflow

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

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