Remove spectrograms from Python 2 also [#5594].

For 0.10.0 we're removing specrograms entirely for reliability. This change only
disables spectrograms and updates install docs and Dockerfiles. We still need
to strip out all the spectrogram code and extlib/freesound/.

mediagoblin/media_types/audio/transcoders.py

@@ -43,15 +43,14 @@ gi.require_version('Gst', '1.0')
 from gi.repository import GObject, Gst
 Gst.init(None)
 
-import numpy
-import six
-
 
+# TODO: Now unused - remove.
 class Python2AudioThumbnailer(object):
     def __init__(self):
         _log.info('Initializing {0}'.format(self.__class__.__name__))
 
     def spectrogram(self, src, dst, **kw):
+        import numpy
         # This third-party bundled module is Python 2-only.
         from mediagoblin.media_types.audio import audioprocessing
 
@@ -113,8 +112,8 @@ class Python2AudioThumbnailer(object):
         th.save(dst)
 
 
-class Python3AudioThumbnailer(Python2AudioThumbnailer):
-    """Dummy thumbnailer for Python 3.
+class DummyAudioThumbnailer(Python2AudioThumbnailer):
+    """A thumbnailer that just outputs a stock image.
 
     The Python package used for audio spectrograms, "scikits.audiolab", does not
     support Python 3 and is a constant source of problems for people installing
@@ -133,7 +132,9 @@ class Python3AudioThumbnailer(Python2AudioThumbnailer):
         img.save(dst)
 
 
-AudioThumbnailer = Python3AudioThumbnailer if six.PY3 else Python2AudioThumbnailer
+# Due to recurring problems with spectrograms under Python 2, and the fact we're
+# soon dropping Python 2 support, we're disabling spectrogram thumbnails. See #5594.
+AudioThumbnailer = DummyAudioThumbnailer
 
 
 class AudioTranscoder(object):