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Maintenance release.
- Drop support for Python 3.3 and 3.4, as both has reached end of life.
- Add support for Python 3.6 and 3.7. No changes was required, but the test
suite now runs on these versions too.
- On Python 3, import :class:`Iterable`, :class:`MutableSequence`, and
:class:`Sequence` from :mod:`collections.abc` instead of :mod:`collections`.
This fixes a deprecation warning on Python 3.7 and prepares for Python 3.8.
- Document that the search API is broken. If it is used, raise an exception
instead of sending the search to Spotify, as that seems to disconnect your
session. (Fixes: :issue:`183`)
- Format source code with Black.