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_xml/__init__.py

@@ -1,7 +1,18 @@
-from .invoice import *
-from .credit_note import *
-from .debit_note import *
-from .utils import *
-from .attached_document import *
-from .support_document import *
-from .support_document_credit_note import *
+from .invoice import DIANInvoiceXML
+from .credit_note import DIANCreditNoteXML
+from .debit_note import DIANDebitNoteXML
+from .utils import DIANWrite, DIANWriteSigned
+from .attached_document import AttachedDocument
+from .support_document import DIANSupportDocumentXML
+from .support_document_credit_note import DIANSupportDocumentCreditNoteXML
+
+__all__ = [
+    'DIANInvoiceXML',
+    'DIANCreditNoteXML',
+    'DIANDebitNoteXML',
+    'DIANWrite',
+    'DIANWriteSigned',
+    'AttachedDocument',
+    'DIANSupportDocumentXML',
+    'DIANSupportDocumentCreditNoteXML',
+]