Update llbase JSON logging for Python 3.13 compatibility

llbase.lljsonlog.JsonFormatter currently imports and supports cgitb for exception formatting. The cgitb module was removed in Python 3.13, so any code path that imports llbase.lljsonlog can fail or require local workarounds when running on Python 3.13.

We recently worked around this in the Join Django app by adding an internal JSON log formatter that is compatible with llbase.lljsonlog.JsonFormatter but does not import cgitb. That workaround keeps Join unblocked, but the underlying compatibility issue still exists in the shared python-llbase library.

Current library location:

llbase/lljsonlog.py

Current problem areas:

• import cgitb happens at module import time.
• _fmt_exc_cgitb() depends on cgitb.Hook.
• JsonFormatter(exception_formatter="cgitb") cannot be supported on Python 3.13 without a replacement implementation.
• The module still carries Python 2 compatibility code, which may make the Python 3.13 update harder to reason about.

Expected outcome:

• llbase.lljsonlog.JsonFormatter should be importable on Python 3.13.
• The default traceback formatter should continue to work.
• Existing users of JsonFormatter() should not need app-level replacements.
• If exception_formatter="cgitb" is still needed, either provide a Python 3.13-safe replacement or clearly document/deprecate that option.
• Add or update tests covering import and basic formatting behavior on supported Python versions.

This should let Join remove its local compatibility formatter once the shared library has a Python 3.13-compatible release.

从 llbase/lljsonlog.py 开始，然后检查模块级别的 cgitb import、_fmt_exc_cgitb()，以及 JsonFormatter 的默认 formatter 路径和 cgitb formatter 路径。检查现有的测试布局，确认对 import 和基本格式化行为的覆盖情况。完成的标准是：模块可以在 Python 3.13 上导入，默认格式化仍然正常工作，并且 cgitb 选项已被安全替换或明确标记为已弃用。