v0.6.0 stable release

[cdfmlr]

@runfengtsui Please conduct an end-to-end test on v0.6.0b1. Once e2e testing is completed, we can proceed with the release of the next stable major version (v0.6.0).

• Perform testing on v0.6.0b1 @runfengtsui: https://pypi.org/project/pyflowchart/0.6.0b1/
• Confirm test results @runfengtsui
• Release v0.6.0 stable @cdfmlr

[cdfmlr]

#41 https://github.com/cdfmlr/pyflowchart/releases/tag/v0.6.0