sagemathgh-39220: some care in src/sage/graphs/comparability.pyx

    
With this PR, we fix many small coding style issues in
`src/sage/graphs/comparability.pyx`. We also do a minor modification to
method `greedy_is_comparability` that was failing for graphs with
vertices of different types.

### 📝 Checklist

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preview.

### ⌛ Dependencies

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URL: sagemath#39220
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton