Interactive debugger console crashes on syntax errors on Python 3.13+

[amol-]

Goal

Make the interactive debugger console render a syntax error page instead of
raising, on Python 3.13 and newer.

Done when: evaluating an invalid expression (for example 1 +) in the
debugger console returns the rendered "Syntax Error" traceback HTML on every
supported Python version (3.9 through 3.14), and a test covers it.

Reproduction:

from backlash.console import Console
Console({}, {}, None).eval('1 +')

Current behaviour:

• Python 3.9 - 3.12: returns the expected <h3>Syntax Error</h3> page.
• Python 3.13 / 3.14: raises
  TypeError: _InteractiveConsole.showsyntaxerror() got an unexpected keyword argument 'source'.

Cause: _InteractiveConsole.showsyntaxerror overrides the stdlib method with
the signature (self, filename=None). Since Python 3.13,
code.InteractiveInterpreter.runsource calls
self.showsyntaxerror(filename, source=source), so the override no longer
matches its caller.

Why

Any user who mistypes an expression in the in-browser debugger console gets an
unhandled exception instead of the error page, on the two most recent Python
releases. Typing a malformed expression is a routine action in an interactive
console, so this is likely to be hit on first use.

No test currently covers this path, so CI is green despite the breakage.

References

• backlash/console.py (_InteractiveConsole.showsyntaxerror, _InteractiveConsole.runsource)
• stdlib code.InteractiveInterpreter.runsource signature change in Python 3.13
• tests/test_console.py may be a natural home for a regression test