The beginPath and addPoint methods of PointToSegmentPen already take
extra **kwargs (via BasePointToSegmentPen); only the addComponent method
does not.
This makes it raise TypeError when an unknown keyword argument is passed
(e.g. 'identifier' for UFO3 objects).
For the PointToSegmentPen, which translates between the PointPen and
the SegmentPen API, it is ok if we only use the arguments that have
an equivalent in the SegmentPen API, and silently discard the **kwargs.
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