Keep markers at the track if child_at_time() found something that does not hold markers

[jchen9]

The issue happens when track.child_at_time() found something the does not have markers property, for example, Transition.

It triggered AttributeError: 'opentimelineio._otio.Transition' object has no attribute 'markers' in the line:

                # attach marker to target item
                target_item.markers.append(marker)

The change now checks both None and whether the target_item has the markers attribute.
An example AAF composition and test case demonstrating the issue is also added.

[meshula]

don't think there's a reason to break up a print statement to satisfy a line length rule in this case, such a split won't make the code more readable. #noqa will resolve it

[codecov-commenter]

Codecov Report

Merging #1239 (d4d82e5) into main (46d19ef) will decrease coverage by 0.00%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##             main    #1239      +/-   ##
==========================================
- Coverage   86.15%   86.15%   -0.01%     
==========================================
  Files         196      196              
  Lines       19624    19632       +8     
  Branches     2302     2302              
==========================================
+ Hits        16907    16913       +6     
- Misses       2159     2161       +2     
  Partials      558      558              

Flag	Coverage Δ
py-unittests	86.15% <77.77%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
...elineio_contrib/adapters/tests/test_aaf_adapter.py	98.08% <75.00%> (-0.28%)	⬇️
...neio_contrib/adapters/advanced_authoring_format.py	81.42% <100.00%> (ø)

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