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/__init__.py

@@ -5,9 +5,11 @@
 # from functools import reduce
 # import copy
 
+from __future__ import annotations
+
 import dataclasses
 from dataclasses import dataclass, field
-from datetime import datetime, date
+from datetime import datetime
 # from collections import defaultdict
 import decimal
 from decimal import Decimal
@@ -278,17 +280,6 @@ class Responsability:
                 raise ValueError("code %s not found" % (code))
 
 
-@dataclass
-class TaxScheme:
-    code: str
-    name: str = ''
-
-    def __post_init__(self):
-        if self.code not in codelist.TipoImpuesto:
-            raise ValueError("code not found")
-        self.name = codelist.TipoImpuesto[self.code]['name']
-
-
 @dataclass
 class Party:
     name: str