Modernize python versions (remove py2x) and fix tests, update spec

[sarnold]

• migrate to github actions for CI, add conda recipe/workflow
• fix document processing, update pandoc args and history
• convert doctests and modules to py3
• convert packaging/setup.py to pep517, keep doc processing
• cleanup tox cfg, add coverage, readme status
• add pep8speaks cfg, cleanup warnings, use correct env
• update setup_description.rst for packaging
• set version for test release => 0.6.0 and deploy

[isuruf]

@defunkt, @cjerdonek, is this project abandoned? If so, will you be willing to hand over maintainership?

[c00kiemon5ter]

https://github.com/defunkt/pystache/pull/213