Update flatten_video_tracks.py to use deepcopy.

[jminor]

Link the Issue(s) this Pull Request is related to.

Fixes #811

Summarize your change.

Now the example uses deepcopy when splicing the audio tracks into the output timeline.

Reference associated tests.

There are no tests for the code in the examples/ folder.

[codecov-io]

Codecov Report

Merging #812 into master will decrease coverage by 6.48%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #812      +/-   ##
==========================================
- Coverage   84.02%   77.54%   -6.49%     
==========================================
  Files          74      109      +35     
  Lines        3124     5647    +2523     
==========================================
+ Hits         2625     4379    +1754     
- Misses        499     1268     +769     

Flag	Coverage Δ
#py27	?
#py37	?
#py38	?
#unittests	77.54% <ø> (-6.64%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ntimelineio/opentimelineio-bindings/otio_utils.cpp	86.90% <0.00%> (-0.46%)	⬇️
src/opentimelineio/typeRegistry.cpp	84.84% <0.00%> (-0.45%)	⬇️
...entimelineio-bindings/otio_serializableObjects.cpp	95.11% <0.00%> (-0.31%)	⬇️
src/opentimelineio/item.cpp	90.47% <0.00%> (-0.15%)	⬇️
...lineio/opentime-bindings/opentime_rationalTime.cpp	97.36% <0.00%> (-0.07%)	⬇️
src/opentimelineio/serialization.cpp	83.60% <0.00%> (-0.06%)	⬇️
src/opentimelineio/timeline.h	100.00% <0.00%> (ø)
src/opentimelineio/typeRegistry.h	100.00% <0.00%> (ø)
...imelineio/opentime-bindings/opentime_timeRange.cpp	100.00% <0.00%> (ø)
...melineio/opentimelineio-bindings/otio_bindings.cpp	100.00% <0.00%> (ø)
... and 46 more

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[jminor]

@ssteinbach do you have any clue about why the code coverage system is confused by this? The file I changed isn't even run by the unit tests, so I don't see how the coverage could drop by any amount, let alone 6.48%

[ssteinbach]

Thats probably a bug in coverage, which happens sometimes. I'll try kicking the CI and see if the new report is correct?