* Move most CI to GitHub Actions
* Build sdist
* Build manylinux1 wheels with libyaml ext (also tested with 2010 and 2014)
* Build MacOS x86_64 wheels with libyaml ext
* Windows wheel builds remain on AppVeyor until we drop 2.7 support in 6.0
* Smoke tests of all post-build artifacts
* Add PEP517/518 build declaration (pyproject.toml with setuptools backend)
* Fully move build to setuptools
* Drop Python 3.5 support
* Declare Python 3.9 support
* Update PyPI metadata now that setuptools lets it flow through
Co-authored-by: Matt Davis <mrd@redhat.com>
* builds Windows wheels against a specified libyaml repo/refspec for many Python versions
* since we don't have multiple Appveyor workers, it's faster/more convenient to run them serially
* not all paths sufficient for general CI usage yet; still needs manual inspection/testing of output
* various hacks to quiet warning noise during build on old Pythons
