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_client_dian.py

@@ -4,12 +4,7 @@
 # this repository contains the full copyright notices and license terms.
 
 from facho.fe.client import dian
-from facho import facho
 
-import pytest
-
-
-from datetime import datetime
 
 class FakeDianClient(dian.DianClient):
     def __init__(self, username, password, resp):