Bug 1843902 - Vendor `PyYAML` at version `6.0.1` a=pascalc

There's a `Cython` issue (https://github.com/yaml/pyyaml/issues/601) with `PyYAML` that breaks our `./mach vendor python` in versions of PyYAML `>5.3.1` and `=<6.0`. This issue has been resolved with `PyYAML` version `6.0.1`, and we can just safely upgrade to it (rather than downgrading to `5.3.1` (which has other issues).

Original Revision: https://phabricator.services.mozilla.com/D183818

Differential Revision: https://phabricator.services.mozilla.com/D200108