Fix memory leak in GMPy_CTXT_Exit()

[skirpichev]

Closes #515

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.61%. Comparing base (224745b) to head (13efb8a).
Report is 46 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
+ Coverage   85.52%   85.61%   +0.08%     
==========================================
  Files          50       50              
  Lines       11656    11657       +1     
  Branches     2202     2202              
==========================================
+ Hits         9969     9980      +11     
+ Misses       1687     1677      -10     

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[casevh]

Thanks for the quick fix.