fix: parenthesize multiple exception types in greatest_common_divisor

[mithunbabbira]

Summary

Fixes #14929 — maths/greatest_common_divisor.py uses Python 2 exception
syntax which is a SyntaxError in Python 3.

Problem

Line 76 used the Python 2 multi-exception form:

except IndexError, UnboundLocalError, ValueError:

This raises SyntaxError on Python 3, so the module cannot be imported.

Root Cause

Python 3 requires parenthesizing multiple exception types.

Solution

except (IndexError, UnboundLocalError, ValueError):

Testing

• python3 -m py_compile maths/greatest_common_divisor.py — passes
• python3 -m doctest maths/greatest_common_divisor.py -v — 6 tests pass

Notes

Minimal, single-line change. No behavior change.

Fixes #14929

[algorithms-keeper]

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