awscli is the only pyyaml revdep that still require a 3.x version, which is soon going to be cleaned up as part of bug 659348. Please update its pyyaml requirements.
It looks like pyyaml 5.1 has been supported since version 1.16.178 and pyyaml 5.2 has been supported since 1.16.219 https://github.com/aws/aws-cli/pull/4231/commits/73b6ece69c1e238a43773203382a4ed162e33905 https://github.com/aws/aws-cli/pull/4400/commits/b7e304560cc4bd7cfd086cb8d450f7f1b9458cbb
This looks obsolete, current versions work with up to pyyaml 5.2.