Fix dropEvent bug

[Joao-Dionisio]

It seems to fix the problem with Issue #820, but the underlying cause might still be there. Me and Mo couldn't figure out why self.model was deleted when the call to dropEvent is made.

Fix #820.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 51.85%. Comparing base (9e16f75) to head (03fec73).

Files	Patch %	Lines
src/pyscipopt/scip.pxi	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
- Coverage   52.18%   51.85%   -0.34%     
==========================================
  Files          18       18              
  Lines        3888     3890       +2     
==========================================
- Hits         2029     2017      -12     
- Misses       1859     1873      +14     

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