Update pyyaml otherwise pip install will fail

[are-agilebg]

With the current version of pyyaml the script travis_install_nightly will fail at line https://github.com/OCA/maintainer-quality-tools/blob/7d99e8d90a9f493c697cbb86d2d8176814fc0f56/travis/travis_install_nightly#L109C5-L109C6 with the following error:

` error: subprocess-exited-with-error

× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> [62 lines of output]
/tmp/pip-build-env-esorkkhj/overlay/lib/python3.10/site-packages/setuptools/config/setupcfg.py:293: _DeprecatedConfig: Deprecated config in setup.cfg
!!

          ********************************************************************************
          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.
          ********************************************************************************
  
  !!`

By updating pyyaml to 6.0 the problem is resolved.