Python 3.9 testing and gzip headers
Added gzip as default request header (PR from @rsaggino )
Update ruamel.yaml to 0.17.16
Update file read/write to explicitly handle encoding per best practices; default to UTF8; configurable in nipyapi.config
Linting and style changes to update to Python 3.9 standards, except where it would break backwards compatibility to Python 2.7
Test support for Amazon Linux 2, mostly better error handling for filesystem responses