[FIX] util/helpers: do not override docs in Python2

We cannot write to `__doc__` attributes of `namedtuple`s in Python2
```
Python 2.7.18 (default, Oct 15 2023, 16:43:11)
[GCC 11.4.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import namedtuple
>>> x = namedtuple("X", "x")
>>> x.__doc__ = "x"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: attribute '__doc__' of 'type' objects is not writable
```

closes #101

Signed-off-by: Christophe Simonis (chs) <[email protected]>

Author: aj-fuentes

src/util/helpers.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 import logging
 import os
+import sys
 from collections import namedtuple
 
 import lxml
@@ -218,15 +219,16 @@ def _get_theme_models():
 
 
 FieldsPathPart = namedtuple("FieldsPathPart", "field_model field_name relation_model")
-FieldsPathPart.__doc__ = """
+if sys.version_info[0] >= 3:
+    FieldsPathPart.__doc__ = """
 Encapsulate information about a field within a fields path.
 
 :param str field_model: model of the field
 :param str field_name: name of the field
 :param str relation_model: target model of the field, if relational, otherwise ``None``
 """
-for _f in FieldsPathPart._fields:
-    getattr(FieldsPathPart, _f).__doc__ = None
+    for _f in FieldsPathPart._fields:
+        getattr(FieldsPathPart, _f).__doc__ = None
 
 
 def resolve_model_fields_path(cr, model, path):