Remove Feature support

Error on warnings.

Try building docs with python3.

Reword UserWarning for insecure extraction path so that the message can be keyed for ignoring it. Ref #1899.

Remove module importing hack

Add info message on package index auth error 

Include pkg_resources test data in sdist

Fix broken link to distutils docs for package_data

Use CPython 3.8.0 mechanism to find msvc 14+

Add news fragment

The broken link used to point to Python 2 docs (which has been moved).
Since Python 2 has reached its end-of-life, link to Python 3 docs instead.

Copy docs requirements to a separate file due to RTD constraints.

Copy docs requirements to a separate file due to RTD constraints. Fixes #2001. Ref readthedocs/readthedocs.org#6662

Pin virtualenv to <20 as workaround for #1998.

Avoid installing setuptools 45 on Python 2.

Because Requires-Python is disabled for Python 2 to allow this declared-incompatible version to install on Python 2, the build-requirements must exclude installing setuptools 45 during the build-system setup.

Because Python 2 tests are only needed for internal validation, and because the latest pip is already installed, the build-system can rely on `backend-path = ["."]`.

Fixes #1996.