Fix: Formateo PEP8 (79 cols) y limpieza flake8 en paquete facho

- Líneas >79 cols reajustadas en todo el paquete
- Fix F821: etree.cleanup_namespaces(security) en signature.py (era 'header', NameError)
- Fix bug cune_xpath -> xpath en nomina/informacion_general() (NameError)
- Elimina clase TaxScheme duplicada en form/__init__.py
- Imports explícitos (F403/F405) y __all__ en form_xml/invoice.py, nomina
- Elimina imports y variables muertas (F401/F841), E712 (== None -> is None)

facho/fe/form/query.py

@@ -6,16 +6,19 @@ from .. import form
 from ..fe import fe_from_string
 from datetime import datetime
 
-def billing_reference(xmldocument: str, klass: form.BillingReference) -> form.BillingReference:
+
+def billing_reference(
+        xmldocument: str,
+        klass: form.BillingReference) -> form.BillingReference:
     """
     construye BillingReference desde XMLDOCUMENT
     usando KLASS como clase.
     """
     if not issubclass(klass, form.BillingReference):
         raise TypeError('klass expected subclass of BillingReference')
-    
+
     fachoxml = fe_from_string(xmldocument)
-    
+
     uid = fachoxml.get_element_text('./cbc:ID')
     uuid = fachoxml.get_element_text('./cbc:UUID')
     issue_date = fachoxml.get_element_text('./cbc:IssueDate')