Improve test coverage

[JeanChristopheMorinPerso]

This PR adds test coverage for the AnyVector and AnyDictionary python bindings and the opentimelineio.core._core_utils module.

It supersedes #1433.

I also change the builds a little bit so that the coverage is measured in debug mode to improve the accuracy of the C++ code coverage when running the python tests and I also added --rc lcov_branch_coverage=1 to the lcov commands.

[codecov-commenter]

Codecov Report

Merging #1436 (811ee44) into main (170dae2) will decrease coverage by 7.54%.
The diff coverage is 94.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1436      +/-   ##
==========================================
- Coverage   86.38%   78.84%   -7.55%     
==========================================
  Files         201      203       +2     
  Lines       20860    22259    +1399     
  Branches     2298     4522    +2224     
==========================================
- Hits        18020    17549     -471     
- Misses       2244     2437     +193     
- Partials      596     2273    +1677     

Flag	Coverage Δ
py-unittests	78.84% <94.50%> (-7.55%)	⬇️

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

Impacted Files	Coverage Δ
...ineio/opentimelineio-bindings/otio_anyDictionary.h	78.00% <0.00%> (-4.93%)	⬇️
...imelineio/opentimelineio-bindings/otio_anyVector.h	59.61% <0.00%> (+9.61%)	⬆️
...-opentimelineio/opentimelineio/core/_core_utils.py	84.12% <ø> (+2.14%)	⬆️
...ntimelineio/opentimelineio-bindings/otio_tests.cpp	28.72% <50.00%> (-41.70%)	⬇️
tests/test_core_utils.py	98.75% <98.75%> (ø)
tests/test_opentime.py	99.74% <100.00%> (+<0.01%)	⬆️
tests/test_serializable_collection.py	97.46% <100.00%> (+0.06%)	⬆️
...elineio/opentimelineio-bindings/otio_anyVector.cpp	30.76% <0.00%> (-69.24%)	⬇️
...entimelineio-bindings/otio_serializableObjects.cpp	24.72% <0.00%> (-66.85%)	⬇️
...timelineio/opentime-bindings/opentime_bindings.cpp	33.33% <0.00%> (-66.67%)	⬇️
... and 64 more

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[ssteinbach]

Small stuff, looks pretty good. Thanks @JeanChristopheMorinPerso!