Fix paga exact reproducibility

[ivirshup]

Fixes #1859

igraph uses python's random state, not numpy's.

TODO:

• Update saved examples for tests
• Release note

[codecov]

Codecov Report

Merging #1922 (4e0f21c) into master (1b74015) will decrease coverage by 0.00%.
The diff coverage is 83.33%.

❗ Current head 4e0f21c differs from pull request most recent head 92cbaae. Consider uploading reports for the commit 92cbaae to get more accurate results

@@            Coverage Diff             @@
##           master    #1922      +/-   ##
==========================================
- Coverage   71.63%   71.62%   -0.01%     
==========================================
  Files          92       92              
  Lines       11248    11248              
==========================================
- Hits         8057     8056       -1     
- Misses       3191     3192       +1     

Impacted Files	Coverage Δ
scanpy/plotting/_tools/paga.py	70.98% <83.33%> (+0.30%)	⬆️
scanpy/cli.py	91.30% <0.00%> (-1.45%)	⬇️
scanpy/tools/_rank_genes_groups.py	93.87% <0.00%> (-0.33%)	⬇️