[Issue #4276] Upgrade poster and geonode/tests for Python 2.7/3 compatibility

[mtnorthcott]

PR to update tests module to be Python 2.7/3 cross-compatible as part of #4276. This includes the upgrade to the now deprecated poster dependency used for testing file uploads. I have replaced this package with requests and requests-toolbelt which have similar multipart capability.

Checklist

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[codecov]

Codecov Report

Merging #5405 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5405   +/-   ##
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  Coverage   57.55%   57.55%           
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  Files         172      172           
  Lines       19264    19264           
  Branches     2996     2996           
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  Hits        11087    11087           
  Misses       7156     7156           
  Partials     1021     1021