Support nested audio dissolve

[markreidvfx]

This fixes an odd structure I've found in a few timelines generated from Media Composer.
It's a Transition inside an OperationGroup

It just ends up becoming a dissolve on the end of a audio clip.

I'm not entirely sure how Media Composer creates this structure, but I've run into on a few timelines.
Without the the fix the adapter raise this exception:

  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 1574, in read_from_file
    result = _transcribe(mobs_to_transcribe, parents=list(), edit_rate=None)
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 821, in _transcribe
    result.append(_transcribe(child, parents + [item], edit_rate, indent + 2))
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 413, in _transcribe
    track = _transcribe(slot, parents + [item], edit_rate, indent + 2)
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 639, in _transcribe
    child = _transcribe(item.segment, parents + [item], edit_rate, indent + 2)
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 631, in _transcribe
    result = _transcribe_operation_group(item, parents, metadata,
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 1148, in _transcribe_operation_group
    child = _transcribe(segment, parents + [item], edit_rate, indent)
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 625, in _transcribe
    child = _transcribe(component, parents + [item], edit_rate, indent + 2)
  File "d:/dev/otio-aaf-adapter/src/otio_aaf_adapter\adapters\advanced_authoring_format.py", line 621, in _transcribe
    metadata["PhysicalTrackNumber"] = list(parent.slots).index(item) + 1
AttributeError: 'Sequence' object has no attribute 'slots'

The fix is to only set PhysicalTrackNumber if the parent is a MobSlot

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.17%. Comparing base (74298e1) to head (9dce4c3).
Report is 4 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   90.12%   90.17%   +0.04%     
==========================================
  Files           3        3              
  Lines        1124     1109      -15     
==========================================
- Hits         1013     1000      -13     
+ Misses        111      109       -2     

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[roger-sacilotto]

FYI, this structure is the result of a track effect that applies pan to the original audio timeline sequence, thus the nesting.

[markreidvfx]

I rebased this on top of the current main. It will cleanly merge now