Issue with testing my function time converter

I'm trying to test my function but for me tests act unexpected way

def time_converter(time_zone=None, date_time=None):
    if not time_zone and not date_time:
        return None
    else:
        from_dt = datetime.strptime(date_time, '%Y-%m-%dT%H:%M:%S.%f')
        time_zone = pytz.timezone(time_zone)
        from_dt = from_dt.astimezone(time_zone)
        from_dt = from_dt.strftime('%Y-%m-%dT%H:%M:%S.%f%z')
        date_time = from_dt[:23]
        time_zone = from_dt[-5:]
        return date_time + time_zone

Here is my tests

class TestTimeConverter(unittest.TestCase):

    def test_none_datetime_timezone_converter(self):
        result = time_converter()
        self.assertIsNone(result)

    def test_ok_datetime_timezone_converter(self):
        time_zone = 'Asia/Bishkek'
        test = '2018-07-26T06:00:00.001+0600'
        input = '2018-07-26T06:00:00.001'
        result = time_converter(time_zone, input)
        self.assertEqual(result, test)

if i run tests on my local machine everthing is ok but when i push my code on the gitlab thiss issue apeare

def test_ok_datetime_timezone_converter(self):
    time_zone = 'Asia/Bishkek'
    test = '2018-07-26T06:00:00.001+0600'
    input = '2018-07-26T06:00:00.001'
    result = time_converter(time_zone, input)

  self.assertEqual(result, test)

E AssertionError: '2018-07-26T12:00:00.001+0600' != '2018-07-26T06:00:00.001+0600' E - 2018-07-26T12:00:00.001+0600 E ? ^^ E + 2018-07-26T06:00:00.001+0600 E ? ^^

 tests/test_time_converter.py:16: AssertionError
 ====================== 1 failed, 1 passed in 0.07 seconds ======================
 ERROR: Job failed: exit code 1

I tried to mock this class to return needed value but E TypeError: can't set attributes of built-in/extension type 'datetime.datetime'