Fix CI & cleanup files

[greyli]

• Fix CI build error.
• Remove old CI files.

[greyli]

The CI error is strange. The 2to3 library required by slimit should exist in Python 3.12.

Using Python 3.12.3 environment at: .tox/py312
  × Failed to build `slimit==0.8.1`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit
      status: 1)

      [stdout]
      Python 3.X support requires the 2to3 tool.

According to 2to3 docs:

Deprecated since version 3.11, will be removed in version 3.13: The lib2to3 module was marked pending for deprecation in Python 3.9 (raising PendingDeprecationWarning on import) and fully deprecated in Python 3.11 (raising DeprecationWarning). The 2to3 tool is part of that. It will be removed in Python 3.13.

[greyli]

The Python 3.12 installed by uv seems to be broken:

py312: commands[0]> python -c 'import lib2to3'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'lib2to3'

https://github.com/miracle2k/webassets/actions/runs/16245351988/job/45867513360?pr=558