Fix timezone bug when converting datetime object to literal

Edm.DateTime should not contain timezone information as it did with
previous implementation. Detail reasoning behind these changes can be
found in commit #adf376f3436b2fdaebb6fc6da65c35664f8b9b3c

Previous output of to_literal:
'2019-10-09T12:14:25.216435+00:00'

New output of to_literal:
'2019-10-09T12:15:39.744821'

Documentation also provide extensive explanation of differences between
DateTime and DateTimeOffset:
https://www.odata.org/documentation/odata-version-2-0/overview/

Author: mamiksik

pyodata/v2/model.py

@@ -377,7 +377,8 @@ def to_literal(self, value):
             raise PyODataModelError(
                 'Cannot convert value of type {} to literal. Datetime format is required.'.format(type(value)))
 
-        return super(EdmDateTimeTypTraits, self).to_literal(value.isoformat())
+        # Sets timezone to none to avoid including timezone information in the literal form.
+        return super(EdmDateTimeTypTraits, self).to_literal(value.replace(tzinfo=None).isoformat())
 
     def to_json(self, value):
         if isinstance(value, str):

tests/test_model_v2.py

@@ -1,7 +1,7 @@
 """Tests for OData Model module"""
 # pylint: disable=line-too-long,too-many-locals,too-many-statements,invalid-name, too-many-lines, no-name-in-module, expression-not-assigned, pointless-statement
 
-from datetime import datetime
+from datetime import datetime, timezone
 from unittest.mock import patch
 import pytest
 from pyodata.v2.model import Schema, Typ, StructTypeProperty, Types, EntityType, EdmStructTypeSerializer, \
@@ -416,7 +416,7 @@ def test_traits_datetime():
 
     # 1. direction Python -> OData
 
-    testdate = datetime(2005, 1, 28, 18, 30, 44, 123456)
+    testdate = datetime(2005, 1, 28, 18, 30, 44, 123456, tzinfo=timezone.utc)
     assert typ.traits.to_literal(testdate) == "datetime'2005-01-28T18:30:44.123456'"
 
     # without miliseconds part