Fix changing leaf after reading

[ghost]

Fixes #733

[codecov]

Codecov Report

Merging #744 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #744      +/-   ##
==========================================
+ Coverage   80.53%   80.56%   +0.02%     
==========================================
  Files         171      171              
  Lines       16458    16449       -9     
  Branches     1495     1492       -3     
==========================================
- Hits        13255    13252       -3     
+ Misses       2835     2829       -6     
  Partials      368      368

Impacted Files	Coverage Δ
...hon/core/tests/test_sanity_type_mismatch_errors.py	93.67% <ø> (+5.89%)	⬆️
sdk/python/core/ydk/types/py_types.py	77.41% <ø> (-0.59%)	⬇️
sdk/python/core/tests/test_sanity_types.py	89.51% <100%> (+0.41%)	⬆️

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