Proper support of pyyaml dependency

[briancurtin]

This currently only works properly if you either pip install requirements.txt directly from a checkout or if you've done the requisite pip command to install from GitHub before or after pip install pine. This is due to yaml/pyyaml#126 and it not yet supporting 3.7.

[hugovk]

Good news: PyYAML 4.1 has been released which now installs successfully on Python 3.7.

yaml/pyyaml#126 (comment)