Refactor packaging

[brocksam]

This PR refactors how Pyproprop is package to conform to PEP 517/PEP 518. This has removed setup.py in favour of pyproject.toml and setup.cfg.

This PR has also moved CI over to GitHub Actions (from AppVeyor and TravisCI). It has created four workflows:

• test.yml: run the test suite on all pushes to the repository;
• ci.yml: run the test suite on a full OS/version matrix on pushes to master and PRs;
• docs.yml: build the docs on all pushes to the repository and PRs; and
• publish.yml: automate the publishing of the package to PyPI.

[codecov[bot]]

Codecov Report

Merging #53 (17812ed) into master (27f8789) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #53   +/-   ##
======================================
  Coverage    95.8%   95.8%           
======================================
  Files           6       6           
  Lines         455     455           
  Branches       81      81           
======================================
  Hits          436     436           
  Misses         14      14           
  Partials        5       5