python: update pyyaml to 6.0.0

[uniquefine]

Update the pyyaml dependency to 6.0.0.
6.0.0 includes this commit that removes the use of the deprecated license_file field in the setup.cfg.

This addresses the following warning:

The license_file parameter is deprecated, use license_files instead.
      
By 2023-Oct-30, you need to update your project and remove deprecated calls
or your builds will no longer be supported.
      
See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.

---

This change is 

[matzf]

Thanks!

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed (commit messages unreviewed), all discussions resolved (waiting on @uniquefine)

[uniquefine]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @uniquefine)

[matzf]

Note: while the update pyyaml was the right thing to fix the failing builds, the license_file deprecation warning was a red herring. The actual error was the stacktrace just below:

      Traceback (most recent call last):
        File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/43ab9e1896251f6982e33e72692349be/external/pypi__pip/pip/_vendor/pep517/in_process/_in_process.py", line 363, in <module>
          main()
        File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/43ab9e1896251f6982e33e72692349be/external/pypi__pip/pip/_vendor/pep517/in_process/_in_process.py", line 345, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
        File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/43ab9e1896251f6982e33e72692349be/external/pypi__pip/pip/_vendor/pep517/in_process/_in_process.py", line 130, in get_requires_for_build_wheel
          return hook(config_settings)
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/build_meta.py", line 341, in get_requires_for_build_wheel
          return self._get_build_requires(config_settings, requirements=['wheel'])
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/build_meta.py", line 323, in _get_build_requires
          self.run_setup()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/build_meta.py", line 338, in run_setup
          exec(code, locals())
        File "<string>", line 271, in <module>
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/__init__.py", line 107, in setup
          return distutils.core.setup(**attrs)
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/core.py", line 185, in setup
          return run_commands(dist)
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/core.py", line 201, in run_commands
          dist.run_commands()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/dist.py", line 969, in run_commands
          self.run_command(cmd)
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/dist.py", line 1234, in run_command
          super().run_command(command)
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/dist.py", line 988, in run_command
          cmd_obj.run()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/command/egg_info.py", line 314, in run
          self.find_sources()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/command/egg_info.py", line 322, in find_sources
          mm.run()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/command/egg_info.py", line 551, in run
          self.add_defaults()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/command/egg_info.py", line 589, in add_defaults
          sdist.add_defaults(self)
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/command/sdist.py", line 104, in add_defaults
          super().add_defaults()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/command/sdist.py", line 251, in add_defaults
          self._add_defaults_ext()
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/command/sdist.py", line 336, in _add_defaults_ext
          self.filelist.extend(build_ext.get_source_files())
        File "<string>", line 201, in get_source_files
        File "/tmp/pip-build-env-_npw2glq/overlay/lib/python3.10/site-packages/setuptools/_distutils/cmd.py", line 107, in __getattr__
          raise AttributeError(attr)
      AttributeError: cython_sources

This is related to the recent Cython 3.0.0 release, which is silently picked up during the build of the pyyaml wheel. See the sort of related discussion in yaml/pyyaml#702 (perhaps we should have picked 6.0.1).

This highlights that some internal dependencies are not pinned (which is bad). I currently don't know if this is a mistake on our side, or with rules_python, or somewhere deep in the python wheel-house.