Fix ValueError in _guess_lexer for single-line source files

[bysiber]

_guess_lexer uses str.index() to find the first newline in source code, but index() raises ValueError when the substring isn't found. The subsequent check if new_line_index != -1 only makes sense with str.find(), which returns -1 on miss.

When a traceback includes a frame from a single-line source file (no trailing newline), _guess_lexer crashes with an unhandled ValueError instead of rendering the traceback.

Changed code.index("\n") → code.find("\n") so the existing -1 guard works as intended.

[willmcgugan]

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