Use appropriate reload method for Py2 or Py3 in tank_api_proxy

[jlucas]

We use a setup where the SGTK Pipeline Configuration is on disk, local to each Project. We put install/core/python onto $PYTHONPATH to enable import sgtk. In the process of transitioning to Python 3 we found these incompatible Python 2 reload calls.

The problem could be solved with six.moves.reload_module instead, but I don't know if you want to expect/force the user to have that library in their base environment since it is not part of the standard library.

[codecov]

Codecov Report

Merging #855 (691d2f3) into master (9df4482) will decrease coverage by 6.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
- Coverage   79.39%   72.83%   -6.56%     
==========================================
  Files         177      177              
  Lines       19303    19303              
==========================================
- Hits        15325    14059    -1266     
- Misses       3978     5244    +1266     

Impacted Files	Coverage Δ
python/tank/commands/app_info.py	17.24% <0.00%> (-79.32%)	⬇️
python/tank/bootstrap/import_handler.py	18.18% <0.00%> (-73.74%)	⬇️
python/tank/commands/folders.py	20.00% <0.00%> (-69.10%)	⬇️
python/tank/commands/setup_project.py	17.32% <0.00%> (-66.29%)	⬇️
python/tank/commands/cache_yaml.py	31.57% <0.00%> (-52.64%)	⬇️
python/tank/commands/util.py	37.50% <0.00%> (-37.50%)	⬇️
python/tank/commands/core_localize.py	39.40% <0.00%> (-36.44%)	⬇️
python/tank/commands/unregister_folders.py	12.58% <0.00%> (-36.43%)	⬇️
python/tank/commands/tank_command.py	47.34% <0.00%> (-32.98%)	⬇️
python/tank/commands/install.py	11.38% <0.00%> (-31.71%)	⬇️
... and 42 more

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