data.ref: versioning XML schemas of "factura electrónica"

The contents of the `schemas-xml` directory are separated into
subdirectories, according to the sources used to build the set.
The subdirectories are named considering the source used and the
most recent modification timestamp present between the files in
the set.

Extra:
New enum `cl_sii.base.constants.XmlSchemasVersionEnum` to define
the available XML schema versions

Author: ycouce-cdd

MANIFEST.in

@@ -3,5 +3,6 @@ include LICENSE
 include README.rst
 recursive-include cl_sii *py
 recursive-include cl_sii/data/cte/schemas-json *.schema.json
-recursive-include cl_sii/data/ref/factura_electronica/schemas-xml *.xsd
+recursive-include cl_sii/data/ref/factura_electronica/schemas-xml/2013_02_07_sii_official *.xsd
+recursive-include cl_sii/data/ref/factura_electronica/schemas-xml/2017_10_23_libre_dte *.xsd
 include cl_sii/py.typed

cl_sii/base/constants.py

@@ -3,6 +3,10 @@
 ================
 
 """
+from __future__ import annotations
+
+import enum
+
 import pytz
 
 from cl_sii.libs.tz_utils import PytzTimezone
@@ -11,3 +15,48 @@
 TZ_CL_SANTIAGO: PytzTimezone = pytz.timezone('America/Santiago')
 
 SII_OFFICIAL_TZ = TZ_CL_SANTIAGO
+
+
+class XmlSchemasVersionEnum(enum.Enum):
+    """
+    Enum of "SII XML Schema Version".
+    The version name is selected considering the last updated timestamp
+    of the files in the set
+
+    """
+
+    V2017_10_23_LIBRE_DTE = '2017_10_23_libre_dte'
+    """
+    Incremental update to version V2013_02_07_SII_OFFICIAL, from an unofficial
+    source since the files available on the SII website are outdated with respect
+    to the regulations (and even the documentation PDFs published alongside)
+
+    Source: repository/project "LibreDTE" at https://github.com/LibreDTE/libredte-lib
+
+    Most recent modification timestamp of the XML schemas: 2017-10-23
+    """
+
+    V2013_02_07_SII_OFFICIAL = '2013_02_07_sii_official'
+    """
+    Official schemas of entities related to these domain concepts:
+    - DTE (Documento Tributario Electrónico)
+    - IECV (Información Electrónica de Libros de Compra y Venta)
+    - LCE (Libros Contables Electrónicos)
+    - RTC (Registro de Transferencia de Crédito)
+
+    All the files have been preserved as they were; schemas are in their
+    original text encoding (ISO-8859-1) and have not been modified in the
+    slightest way.
+
+    Sources (2021-04-19):
+    http://www.sii.cl/factura_electronica/schema_dte.zip
+    http://www.sii.cl/factura_electronica/schema_iecv.zip
+    http://www.sii.cl/factura_electronica/schema_cesion.zip
+
+    Most recent modification timestamp of the XML schemas: 2013-02-07
+    """
+
+    @classmethod
+    def latest(cls) -> XmlSchemasVersionEnum:
+        """Reference to the latest version available"""
+        return cls.V2017_10_23_LIBRE_DTE