Reinstate audio spectrograms on Python 3 [#5610].

The `audiotospectrogram` module is a complete rewrite of the existing spectrogram
code with support for Python 3. This allows us to drop the bundled `freesound`
library and Python 2-only `audioprocessing` and `spectrogram` modules.

Signed-off-by: Ben Sturmfels <ben@sturm.com.au>

docs/source/siteadmin/media-types.rst

@@ -92,10 +92,11 @@ as whatever GStreamer plugins you want, good/bad/ugly):
 
     # Debian and co.
     sudo apt install python3-gst-1.0 gstreamer1.0-plugins-{base,bad,good,ugly} \
-    gstreamer1.0-libav
+    gstreamer1.0-libav python3-numpy
 
     # Fedora and co.
-    sudo dnf install gstreamer1-plugins-{base,bad-free,good,ugly-free}
+    sudo dnf install gstreamer1-plugins-{base,bad-free,good,ugly-free} \
+    python3-numpy
 
 Add ``[[mediagoblin.media_types.audio]]`` under the ``[plugins]`` section in your
 ``mediagoblin.ini`` and update MediaGoblin::

docs/source/siteadmin/relnotes.rst

@@ -30,6 +30,7 @@ carefully, or at least skim over it.
 **Improvements:**
 
  - Drop Python 2 installation support (Ben Sturmfels)
+ - Reinstate Python 3 audio spectrograms [#5610] (Fernando Gutierrez)
 
 **Bug fixes:**