Support python 3 in the "rv_session" adapter.

[avrata]

This can be used with rv versions that support python 3

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

Fixes #1264

Summarize your change.

The rv_session adapter was written in a time before any version of rv supported python 3. That's now a supported version of python so this change enables python 3 for this adapter.

Add a list of changes, and note any that might need special attention during review.

Reference associated tests.

There was one issue with testing where the connections list was not stable between python versions. I think this is due to dict order differences used in the rvSession.py's nodes dict.

[codecov-commenter]

Codecov Report

Merging #1267 (392df8e) into main (1d3080e) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1267      +/-   ##
==========================================
- Coverage   86.17%   86.15%   -0.02%     
==========================================
  Files         196      196              
  Lines       19697    19701       +4     
  Branches     2309     2310       +1     
==========================================
  Hits        16973    16973              
- Misses       2162     2166       +4     
  Partials      562      562              

Flag	Coverage Δ
py-unittests	86.15% <0.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
...ntrib/opentimelineio_contrib/adapters/extern_rv.py	0.00% <0.00%> (ø)
contrib/opentimelineio_contrib/adapters/rv.py	15.75% <0.00%> (-0.22%)	⬇️

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[jminor]

Thanks for the fix!