Log Message:
Update py-gstreamer1 to 1.12.3

GStreamer Python Bindings (highlights)

* Overrides has been added for IntRange, Int64Range, DoubleRange,
  FractionRange, Array and List. This finally enables Python
  programmers to fully read and write GstCaps objects.
* Some plugin filenames have been changed to match the plugin names:
  for example the file name of the encoding plugin in
  gst-plugins-base containing the encodebin element was
  libgstencodebin.so and has been changed to libgstencoding.so. This
  affects only a handful of plugins across modules.
* GStreamer core now optionally uses libunwind and libdw to generate
  backtraces. This is useful for tracer plugins used during debugging
  and development.
* There is a new libgstbadallocators-1.0 library in gst-plugins-bad
  (which may go away again in future releases once the
  GstPhysMemoryAllocator interface API has been validated by more
  users).
* The qtkitvideosrc element for macOS was removed. The API is
  deprecated since 10.9 and it wasn't shipped in the binaries since a
  few releases.