Upgrade to PyYAML 6 + update lockfiles.

[asherf]

PyYAML 6:
https://github.com/yaml/pyyaml/blob/master/CHANGES

6.0 (2021-10-13)

* https://github.com/yaml/pyyaml/pull/327 -- Change README format to Markdown
* https://github.com/yaml/pyyaml/pull/483 -- Add a test for YAML 1.1 types
* https://github.com/yaml/pyyaml/pull/497 -- fix float resolver to ignore `.` and `._`
* https://github.com/yaml/pyyaml/pull/550 -- drop Python 2.7
* https://github.com/yaml/pyyaml/pull/553 -- Fix spelling of “hexadecimal”
* https://github.com/yaml/pyyaml/pull/556 -- fix representation of Enum subclasses
* https://github.com/yaml/pyyaml/pull/557 -- fix libyaml extension compiler warnings
* https://github.com/yaml/pyyaml/pull/560 -- fix ResourceWarning on leaked file descriptors
* https://github.com/yaml/pyyaml/pull/561 -- always require `Loader` arg to `yaml.load()`
* https://github.com/yaml/pyyaml/pull/564 -- remove remaining direct distutils usage