Fix: Formateo PEP8 (79 cols) y limpieza flake8 en tests y examples

- Líneas >79 cols reajustadas (asserts, strings CUDE/CUDS, comentarios)
- Elimina imports y variables muertas (F401/F841), E712 (== False -> is False)
- Reordena imports en docs/conf.py

tests/test_form.py

@@ -121,7 +121,9 @@ def test_FAU14():
 
     wants = form.Amount(119.0 + 19.0 - 50.0)
 
-    assert inv.invoice_legal_monetary_total.payable_amount == wants, "got %s want %s" % (inv.invoice_legal_monetary_total.payable_amount, wants)
+    assert inv.invoice_legal_monetary_total.payable_amount == wants, (
+        "got %s want %s" % (
+            inv.invoice_legal_monetary_total.payable_amount, wants))
 
 
 def test_invalid_tipo_operacion_nota_debito():