beets: Set Python version via top-level attribute

This largely reverts commit 599312739e3a338f923e6c22ab183178ca49c7b6.

The main reason is that it breaks the plugins, because the mentioned
commit didn't change the attributes for the plugins as well.

But instead of just fixing the attributes when we import the plugin
packages, let's just override pythonPackages in all-packages.nix.

Right now, Beets is in transition to Python 3, so we don't need to wait
that long until we can remove the dependency on Python 2:

https://github.com/beetbox/beets/releases/tag/v1.4.1

Once Python 3 support is no longer beta, we can just change this by
changing one line only instead of several.

Tested this by building beets with both external plugins.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @FRidh

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