Fix update_virtual_environment_for_custom_operators

[PProfizi]

Issue with new setuptools behavior which creates a __editable__.ansys_dpf_core-****.pth instead of ansys.dpf.core.pth in the interpreter's site-packages when installing in editable mode.
Also fixed the discovery of any original dpf-sites.zip.

[codecov]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.91%. Comparing base (ec06e9e) to head (d6c1e56).
Report is 7 commits behind head on master.

❗ Current head d6c1e56 differs from pull request most recent head 9f72bc5

Please upload reports for the commit 9f72bc5 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1632      +/-   ##
==========================================
- Coverage   82.20%   79.91%   -2.30%     
==========================================
  Files          83       83              
  Lines        9855     9866      +11     
==========================================
- Hits         8101     7884     -217     
- Misses       1754     1982     +228