Cached tzinfo conflict in tests

[rsb-23]

Issue

Unit tests fail if we change the name of class as below:

class TestIcalendar(unittest.TestCase):

class TestAIcalendar(unittest.TestCase):

Reason

def test_pytz_timezone_serializing is changing the tzinfo for other tests.

Expectation

Testcases should not interfere with each other.

[rsb-23]

[Unrelated] Few doctest failure on running pytest --doctest-modules.