Fixed broken repr and str magic methods for Marker

[reinecke]

It appears the __str__ and __repr__ methods may have been copy/pasted from Clip by accident in the C++ port, printing in the python REPL would result in a traceback like:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ereinecke/.venv/lib/python3.8/site-packages/opentimelineio/schema/marker.py", line 25, in __repr__
    repr(self.media_reference),
AttributeError: 'opentimelineio._otio.Marker' object has no attribute 'media_reference'

Restored implementations from the pre-C++ port version and added unitests to prevent this kind of error in the future.