refact: fix warnings in pytest output

[oakbani]

Summary

• Renames TestEventDispatcher class so that pytest doesn't mistake it for a test class.
• Replace SimpleLogger from event tag utils test since it's been deprecated.

Warnings

tests/test_event_processor.py:49
  /home/oakbani/Documents/Optimizely/python-sdk/tests/test_event_processor.py:49: PytestCollectionWarning: cannot collect test class 'TestEventDispatcher' because it has a __init__ constructor (from: tests/test_event_processor.py)
    class TestEventDispatcher(object):

tests/test_event_processor.py:498
  /home/oakbani/Documents/Optimizely/python-sdk/tests/test_event_processor.py:498: PytestCollectionWarning: cannot collect test class 'TestForwardingEventDispatcher' because it has a __init__ constructor (from: tests/test_event_processor.py)
    class TestForwardingEventDispatcher(object):

tests/helpers_tests/test_event_tag_utils.py::EventTagUtilsTest::test_get_numeric_metric__value_tag
tests/helpers_tests/test_event_tag_utils.py::EventTagUtilsTest::test_get_numeric_metric__value_tag
tests/helpers_tests/test_event_tag_utils.py::EventTagUtilsTest::test_get_numeric_metric__value_tag
tests/helpers_tests/test_event_tag_utils.py::EventTagUtilsTest::test_get_numeric_metric__value_tag
tests/helpers_tests/test_event_tag_utils.py::EventTagUtilsTest::test_get_numeric_metric__value_tag
tests/helpers_tests/test_event_tag_utils.py::EventTagUtilsTest::test_get_numeric_metric__value_tag
  /home/oakbani/Documents/Optimizely/python-sdk/optimizely/logger.py:83: DeprecationWarning: <class 'optimizely.logger.SimpleLogger'> is deprecated. Please use standard python loggers.
    warnings.warn(warning, DeprecationWarning)

Test plan

All tests should continue to pass

Issues

[coveralls]

Coverage remained the same at 97.154% when pulling 2aef4273bc8c615e3b69470ab095192425bc54f2 on oakbani/remove-warnings into ce0827d5fe1b4f7eb314f42c8d7eeaee3eda0a1d on master.