Upper bound networkx version at 2.4

[bhancock8]

Description of proposed changes

Upper bound the version of networkx at 2.4 instead of 3.0, following the discovery of backwards incompatibility described in #1491.

Fixes #1491

Test plan

Regular unit tests pass.

Checklist

Need help on these? Just ask!

• [ ] I have read the CONTRIBUTING document.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] I have run tox -e complex and/or tox -e spark if appropriate.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Merging #1492 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1492   +/-   ##
======================================
  Coverage    97.6%   97.6%           
======================================
  Files          55      55           
  Lines        2049    2049           
  Branches      335     335           
======================================
  Hits         2000    2000           
  Misses         22      22           
  Partials       27      27