beets-alternatives: 0.8.2 -> 0.9.0

This introduces the following upstream changes:

  * The package is now on PyPI
  * Require at least beets v1.4.7
  * Update album art in alternatives when it changes
  * Python 3 support (Python 2.7 continues to be supported)
  * Support the format aliases defined by the convert plugin ('wma' and
    'vorbis' with current beets)
  * Bugfix: Explicitly write tags after encoding instead of relying on
            the encoder to do so
  * Bugfix: If the formats config option is modified, don't move files
            if the extension would change, but re-encode

I updated this because I was pinged by @wisp3rwind about moving back to
@geigerzaehler's repository at [1].

This is what @wisp3rwind wrote in the comment[2] (which was originally
directed to @Profpatsch):

  (I hope you're the one to bug, or at least can ping someone else), I
  just noticed that you switched the NixOS package to my repository.
  Would you please switch it back to this repo soon-ish? The code here
  is better tested, and [3] is handled less elegantly on my fork since
  it requires changes to the configuration. The latter are undocumented,
  but whoever has bothered to take a look at the code might end up with
  (harmless) unused config entries.

So in essence we're now back to the original upstream repository again,
which I changed to @wisp3rwind's fork in 29e89248
because it fixed issues with Python 3.

Stripping the long_description from setup.py also doesn't seem to be
required anymore, but I didn't investigate why (might be because either
our Python tooling now sets a default language or the README simply no
longer has non-ASCII characters).

[1]: https://github.com/geigerzaehler/beets-alternatives
[2]: https://github.com/geigerzaehler/beets-alternatives/issues/23
[3]: https://github.com/geigerzaehler/beets-alternatives/pull/27



Signed-off-by: aszlig <aszlig@nix.build>