Adjust to API change in gst-python >= 1.18

[arybczak]

Without this with gst-python-1.18.2 while scanning files there are intermittent fails as follows:

ERROR    2020-12-18 18:39:25,302 [426578:Core-8] mopidy.core.library
  FileBackend backend caused an exception.
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/mopidy/core/library.py", line 17, in _backend_error_handling
    yield
  File "/usr/lib/python3.9/site-packages/mopidy/core/library.py", line 217, in lookup
    result = future.get()
  File "/usr/lib/python3.9/site-packages/pykka/_threading.py", line 45, in get
    _compat.reraise(*self._data['exc_info'])
  File "/usr/lib/python3.9/site-packages/pykka/_compat/__init__.py", line 29, in reraise
    raise value
  File "/usr/lib/python3.9/site-packages/pykka/_actor.py", line 193, in _actor_loop
    response = self._handle_receive(envelope.message)
  File "/usr/lib/python3.9/site-packages/pykka/_actor.py", line 299, in _handle_receive
    return callee(*message.args, **message.kwargs)
  File "/usr/lib/python3.9/site-packages/mopidy/file/library.py", line 98, in lookup
    result = self._scanner.scan(uri)
  File "/usr/lib/python3.9/site-packages/mopidy/audio/scan.py", line 70, in scan
    tags, mime, have_audio, duration = _process(pipeline, timeout)
  File "/usr/lib/python3.9/site-packages/mopidy/audio/scan.py", line 268, in _process
    tags.update(tags_lib.convert_taglist(taglist))
  File "/usr/lib/python3.9/site-packages/mopidy/audio/tags.py", line 83, in convert_taglist
    if data:
ValueError: operation forbidden on released memoryview object

It's the same problem as fixed in beetbox/audioread#110.

[codecov]

Codecov Report

Merging #1948 (1b738c3) into develop (9ae3520) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #1948      +/-   ##
===========================================
- Coverage    77.03%   76.99%   -0.04%     
===========================================
  Files           55       55              
  Lines         4711     4713       +2     
===========================================
  Hits          3629     3629              
- Misses        1082     1084       +2     

Impacted Files	Coverage Δ
mopidy/audio/tags.py	79.41% <0.00%> (-1.59%)	⬇️

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[kingosticks]

Thanks, this is really helpful.

I don't recall this getting any mention in the the normal GStreamer changelog. I don't know where I am supposed to be looking for breaking API changes like this.

[jodal]

Thanks!