Update CI

[gboeing]

• disable fail fast, so all platforms get tested without canceling if one fails
• add git repack script
• for environment creation, move python-igraph to requirements file now that it's available from conda-forge on windows

[codecov]

Codecov Report

Merging #644 (78163be) into master (839aa9b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #644   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files          24       24           
  Lines        2326     2326           
=======================================
  Hits         2159     2159           
  Misses        167      167           

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