improve: set default format to #%d

Previous « Inconnu_%05d » was an artefact of REPERE evaluation campaign.
fix: remove old constant names

SEGMENT becomes PYANNOTE_SEGMENT.
The same goes for TRACK and LABEL.
refactor(Annotation): remove support for >> operator

BREAKING CHANGE: Timeline tagging is removed.

    One should use pyannote.algorithms.tagging instead.
fix: make label_timeline return a copy

Before this change, adding segments to the returned timeline would
modify the internal _labels attribute!