Allow SyntaxError.msg to be writable and reflected in string formatting by Copilot · Pull Request #6493 · RustPython/RustPython

ls /
Traceback (most recent call last):
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/shells/base_shell.py", line 524, in compile
    compile_empty_tree=False,
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/execer.py", line 131, in compile
    tree = self.parse(input, ctx, mode=mode, filename=filename, transform=transform)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/execer.py", line 87, in parse
    tree, input = self._parse_ctx_free(input, mode=mode, filename=filename)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/execer.py", line 346, in _parse_ctx_free
    return _try_parse(input, greedy=True)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/execer.py", line 344, in _parse_ctx_free
    return _try_parse(input, greedy=False)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/execer.py", line 338, in _try_parse
    input = "\n".join(lines)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/execer.py", line 239, in _try_parse
    debug_level=(self.debug_level >= 2),
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/base.py", line 505, in parse
    tree = self.parser.parse(input=s, lexer=self.lexer, debug=debug_level)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/ply/yacc.py", line 335, in parse
    return self.parseopt_notrack(input, lexer, debug, tracking, tokenfunc)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/ply/yacc.py", line 1203, in parseopt_notrack
    tok = call_errorfunc(self.errorfunc, errtoken, self)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/ply/yacc.py", line 194, in call_errorfunc
    r = errorfunc(token)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/base.py", line 3685, in p_error
    self._parse_error(msg, self.currloc(lineno=p.lineno, column=p.lexpos))
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/base.py", line 632, in _parse_error
    raise_parse_error(msg, loc, self._source, self.lines)
  File "/Users/pc/.local/lib/rustpython3.13/site-packages/xonsh/parsers/base.py", line 212, in raise_parse_error
    err.msg = str(msg) if msg else ""
AttributeError: attribute 'msg' of 'SyntaxError' objects is not writable
```</issue_description>

<agent_instructions>Fix PySyntaxError to allow `msg` setter. Ensure `cargo run -- -m test test_exceptions -vv` success</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

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