Add py3-none-any wheels to the distribution artefacts

[freakboy3742]

Fixes #1912.

• Adds support for generating py3-none-any wheels. If the user has COVERAGE_DISABLE_EXTENSION defined in their environment, no attempt will be made to compile a binary wheel, even if the platform would otherwise support building the binary extension
• Adds a py3-none-any artefact to the build products of a normal build. A wheel was already being compiled as part of the sdist step; this ensures that (a) the wheel is a py3-none-any wheel, and (b) it's included in the release artefacts.

[freakboy3742]

Just realised that if I rename the branch, I can generate a test of the kit workflow; closing and re-opening.