`pyyaml 6.0` fails to build on Python 3.11. There's already an implicit
dependency on `pyyaml==6.0.2`, so pinning it explicitly prevents an
unnecessary attempted downgrade anyway.
Also pin `pyyaml==6.0.2` in mozharness just for consistency.
Differential Revision: https://phabricator.services.mozilla.com/D245175