Obey the connection's TIME_ZONE setting when USE_TZ enabled

Given that SQL server has no built-in support for timezones, we have set
DatabaseFeatures.supports_timezones = False. However, we incorrectly
assumed that every datetime stored in the database is in UTC, and have
hardcoded timezone.utc.

Django allows developers to set the TIME_ZONE option in a database's
connection settings to specify the timezone in which the datetimes are
stored, and this library should obey that setting.

See:
https://docs.djangoproject.com/en/1.11/ref/settings/#time-zone

To obey the TIME_ZONE setting, we replace timezone.utc with
self.connection.timezone, which is a cached_property we inherit from
BaseDatabaseWrapper. The calculation of the property includes various
checks, and will default to timezone.utc if no TIME_ZONE setting is
provided for a database connection - meaning this change will be
backward compatible.

See:
https://github.com/django/django/blob/1.11.4/django/db/backends/base/base.py#L121-L128

Author: justinsg

sql_server/pyodbc/operations.py

@@ -134,7 +134,7 @@ def convert_datetimefield_value(self, value, expression, connection, context):
                 self._warn_legacy_driver('datetime2')
                 value = datetime.datetime.strptime(value[:26], '%Y-%m-%d %H:%M:%S.%f')
             if settings.USE_TZ:
-                value = timezone.make_aware(value, timezone.utc)
+                value = timezone.make_aware(value, self.connection.timezone)
         return value
 
     def convert_floatfield_value(self, value, expression, connection, context):
@@ -480,7 +480,7 @@ def adapt_datetimefield_value(self, value):
             return None
         if settings.USE_TZ and timezone.is_aware(value):
             # pyodbc donesn't support datetimeoffset
-            value = value.astimezone(timezone.utc).replace(tzinfo=None)
+            value = value.astimezone(self.connection.timezone).replace(tzinfo=None)
         if not self.connection.features.supports_microsecond_precision:
             value = value.replace(microsecond=0)
         return value