Initialize Python Imath values

[darbyjohnston]

This PR initializes Imath values created in Python to 0.0. In C++ the default constructor does not initialize the values which means they could be anything. This is done as an optimization for C++ but is probably not necessary and also unexpected in Python.

For example this was recently causing CI failures: #1955

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.11%. Comparing base (04ab39c) to head (045351f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1956   +/-   ##
=======================================
  Coverage   85.10%   85.11%           
=======================================
  Files         181      181           
  Lines       12768    12774    +6     
  Branches     1206     1206           
=======================================
+ Hits        10866    10872    +6     
  Misses       1719     1719           
  Partials      183      183           

Flag	Coverage Δ
py-unittests	85.11% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...ntimelineio/opentimelineio-bindings/otio_imath.cpp	92.59% <100.00%> (ø)
tests/test_box2d.py	96.07% <100.00%> (+0.24%)	⬆️
tests/test_v2d.py	97.56% <100.00%> (+0.09%)	⬆️

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