Remove `pyyaml==6.0` as Direct Dependency

[NimJay]

Describe request or inquiry

• This is regarding the Python services, recommendationservice.
• See https://github.com/GoogleCloudPlatform/microservices-demo/pull/711.
• In the above pull-request, we updated the (indirect) dependency pyyaml to 6.0 by adding it to the requirements.in file (as a "direct" dependency) and specifying the version we want (6.0).
• recommendationservice was only using pyyaml because of google-python-cloud-debugger.
• Eventually, when google-python-cloud-debugger upgrades its version of pyyaml, we will need to upgrade google-python-cloud-debugger and remove pyyaml from requirements.in.

What purpose/environment will this feature serve?

• This is a clean-up.

[bourgeoisor]

I think we should still do this. Commenting to reset SLO.

[mathieu-benoit]

Should be fixed by https://github.com/GoogleCloudPlatform/microservices-demo/pull/985