Passing config_buffer option breaks with PyYAML6 due to extra required parameter

[Hawk777]

In PyYAML 6, the pyyaml.load function started requiring a second parameter. zconfig.YAMLConfig.load_config calls that function if the config_buffer parameter is passed, and does not provide one, thus failing with PyYAML 6.

[amimoto]

Should be resolved

• Added regression test
• Switched to using safe_load rather than pyyaml.load.
• Published v3.0.20230207