Update to 5.1

https://github.com/yaml/pyyaml/blob/5.1/announcement.msg

Announcing PyYAML-5.1

A new MAJOR RELEASE of PyYAML is now available:
https://pypi.org/project/PyYAML/

This is the first major release of PyYAML under the new maintenance team.

Among the many changes listed below, this release specifically addresses the
arbitrary code execution issue raised by:

https://nvd.nist.gov/vuln/detail/CVE-2017-18342

(See https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation
for complete details).
...

PR: 237501
Reported by: sergey@akhmatov.ru