py-yaml: updated to 5.2

5.2:
* Repair incompatibilities introduced with 5.1. The default Loader was changed,
  but several methods like add_constructor still used the old default
  https://github.com/yaml/pyyaml/pull/279 -- A more flexible fix for custom tag constructors
  https://github.com/yaml/pyyaml/pull/287 -- Change default loader for yaml.add_constructor
  https://github.com/yaml/pyyaml/pull/305 -- Change default loader for add_implicit_resolver, add_path_resolver
* Make FullLoader safer by removing python/object/apply from the default FullLoader
  https://github.com/yaml/pyyaml/pull/347 -- Move constructor for object/apply to UnsafeConstructor
* Fix bug introduced in 5.1 where quoting went wrong on systems with sys.maxunicode <= 0xffff
  https://github.com/yaml/pyyaml/pull/276 -- Fix logic for quoting special characters
* Other PRs:
  https://github.com/yaml/pyyaml/pull/280 -- Update CHANGES for 5.1

(adam)