[VR-12461] Bump PyYAML version constraint

[convoliution]

Impact and Context

PyYAML has a security vulnerability that was addressed in pyyaml==5.4. Although verta itself doesn't use that vulnerable code, its dependency version constraint was conflicting with other libraries'.

This PR bumps the upper constraint on our pyyaml dependency from <5.4 to <6.0, which is when Python 2 support is dropped.

Closes #2536.

Risks and Area of Effect

Low risk: From PyYAML's changelog, the new versions we're allowing do not introduce any relevant breakages.

Low area of effect: verta only uses PyYAML for reading user-optional config files (currently only client config & endpoint update config).

Testing

I ran

pytest test_config.py test_endpoint/test_endpoint.py

from my machine on both Python 2 and 3, which encompasses all our config file-related functionality.

There were a handful of unrelated failures:

• FAILED test_endpoint/test_endpoint.py::TestEndpoint::test_update_init_error
  • VR-12994
• FAILED test_endpoint/test_endpoint.py::TestEndpoint::test_update_with_custom_module in Python 2
  • VR-11973
• FAILED test_endpoint/test_endpoint.py::TestEndpoint::test_update_twice
  • updating an endpoint twice in one test exceeds the timeout, and should probably be moved out of our client test suite

How to Revert

Revert this PR.

[liuverta]

Also closed #2379