remove .dev. from version

add python_requires to METADATA, as suggested by H.F. Schreiner

update version in README

update links, release date

update setup.py to workaround issue in setuptools 50.0

update .c to incorporate size_t change for marks

Arnaud Bergeron's change to the .pyx needs to percolate to the _ruamel_yaml.c file
(dv make cython)

Added tag 0.1.1 for changeset 15f91a23adef

prevent -nspkg.pth file from being generated

added appveyor YAML file