fix: IPython 9-only compatibility and color restoration

[tawsifazad23]

Summary

This PR updates ChatDBG’s Pdb integration to work cleanly with IPython 9.x (9.0–9.6).
Older IPython versions (<9) are no longer supported.

Problem

IPython 9 changed its internal debugger API:

• Removed self.color_scheme_table.active_colors (breaks color printing)
• Removed context= parameter from print_stack_entry()

These changes caused:

• AttributeError: 'ChatDBG' object has no attribute 'color_scheme_table'
• TypeError: print_stack_entry() got an unexpected keyword argument 'context'

Fix

• Replaced all use of color_scheme_table with new _resolve_colors() helper.
• Added try/except to safely call print_stack_entry() without context kwarg.
• Updated pyproject.toml → ipython>=9,<10.
• Verified that colors render correctly under IPython 9.6.0.

Validation

• macOS, Python 3.11–3.13
• chatdbg -c continue boom.py → works cleanly, color intact
• why command functions normally
• CHATDBG_NO_COLOR=1 disables color safely

Notes

This officially drops IPython 8.x support to simplify maintenance.
Color output now relies on IPython’s native theming system.

[nicovank]

1. Could you rebase on top of the current trunk? (so it's past the Python version minimum decrease commit)
2. lol, looks like IPython 9+ requires Python 3.10+. So let's just raise this back again. 3.9 was a short stint, but at least people downloading the ChatDBG on 3.9 will get a more recent version (I'll issue a release before merging this).
3. In the pyproject, you can just put ipython>=9.6.0.

I'll take another look and test this out later this week.

[emeryberger]

Subsumed by just-merged PR.