I am sorry to report that the PyYAML-4.1 release from 48 hours ago has been removed from PyPI
There were too many problems to make this a viable release.
The biggest known issue with this retraction is that PyYAML will not work with the new Python 3.7 until PyYAML-4.2 is released. https://github.com/yaml/pyyaml/issues/126#issuecomment-401175258
We are starting work immediately on 4.2b1 prerelease series. I hope to see 4.2 released in the next few days.
Work is being coordinated on #pyyaml on irc.freenode.net and issues can be reported and followed at https://github.com/yaml/pyyaml
Thank you for your patience.