FIX globals for functions defined in __main__

[tomMoral]

Fix #187

The cause of this issue is that global variables defined in __main__ are not shared by unpickled functions and their value are overwritten at each unpickling.

[codecov-io]

Codecov Report

Merging #188 into master will increase coverage by 0.58%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   84.09%   84.68%   +0.58%     
==========================================
  Files           1        1              
  Lines         547      555       +8     
  Branches       99      104       +5     
==========================================
+ Hits          460      470      +10     
+ Misses         63       62       -1     
+ Partials       24       23       -1

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	84.68% <100%> (+0.58%)	⬆️

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