Some fixes, and release task improvements

• Officially supported Python versions now 3.6 .. 3.8
• fix: Handle changed pypandoc API
• fix: Add auto-mode for project layout (close #12)
• Refactored setup.py access and added some poetry support
• new release.shiv task
• releasing: do early and explicit check for pex/shiv
• releasing: use sys.executable for subcalls
• releasing: support 'pex --windows'
• release.pex: handle 3.8 having no more 'm' ABI flag
• release.pex: write archive to dist/, not bin/
• release.pex: ignore any pure wheel (none-any) when it comes to dist renaming
• release.pex: filter out platform tags with a compatible Python ABI before renaming
• git tags: better error reporting when tag splitting fails
• docs: add source to watch dirs when using API docs
• SCM versions: only consider annotated tags, and exclude 'stable'
• tasks requirements: shiv and pex for releasing
• upload_artifacts() is now usable from outside a release task