Community Detection: fix networkx dependency in doctests

[jsiirola]

Fixes # .

Summary/Motivation:

This is a follow-on to (and includes) #2884. This resolves doctest failures on OSX/Python 3.7 due to missing :skiptest: directives in the Community Detection docs.

Changes proposed in this PR:

• Add missing `:skiptest: directives

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01 🎉

Comparison is base (91b249e) 87.46% compared to head (848dc13) 87.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2885      +/-   ##
==========================================
+ Coverage   87.46%   87.47%   +0.01%     
==========================================
  Files         771      771              
  Lines       89651    89651              
==========================================
+ Hits        78410    78419       +9     
+ Misses      11241    11232       -9     

Flag	Coverage Δ
linux	84.52% <ø> (-0.01%)	⬇️
osx	74.59% <ø> (+0.01%)	⬆️
other	84.70% <ø> (+<0.01%)	⬆️
win	82.12% <ø> (+0.01%)	⬆️

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see 3 files with indirect coverage changes

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[bernalde]

Thank you for this, @jsiirola !