Migrate to project.toml based setuptools

[ChenglongMa]

Changelog

v1.2.3

In this version, we have made the following changes:

1. 🧬 CHANGE!: We change the GUI mode to optional.
   • Now, you can install the GUI mode by running:

     • pip install skin-tone-classifier[all] --upgrade

     • It will support both the CLI mode and the GUI mode.
   • If you don't specify the [all] option, the app will install the CLI mode only.
2. 🧬 CHANGE!: [For developer]. We base the project to project.toml instead of setup.py.