Add environment variable to skip doc tes

[ssteinbach]

in some cases you may want to skip the serialized schema tests when running locally. Adds an environment variable to skip that.

[codecov-io]

Codecov Report

Merging #944 (bd38606) into master (cd2b80b) will decrease coverage by 0.13%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #944      +/-   ##
==========================================
- Coverage   85.77%   85.64%   -0.14%     
==========================================
  Files         191      191              
  Lines       18108    18157      +49     
  Branches     2048     2061      +13     
==========================================
+ Hits        15533    15551      +18     
- Misses       2053     2083      +30     
- Partials      522      523       +1     

Flag	Coverage Δ
unittests	85.64% <50.00%> (-0.14%)	⬇️

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Impacted Files	Coverage Δ
tests/test_serialized_schema.py	92.59% <50.00%> (-7.41%)	⬇️
contrib/opentimelineio_contrib/adapters/rv.py	15.97% <0.00%> (-10.96%)	⬇️
...ntrib/opentimelineio_contrib/adapters/extern_rv.py	0.00% <0.00%> (ø)

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