Backport PR #4236 on branch v0.16.x (Migrate to networkx 2.x)

[meeseeksmachine]

Backport PR #4236: Migrate to networkx 2.x

[pep8speaks]

Hello @meeseeksmachine! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file doc/examples/segmentation/plot_rag_merge.py:

Line 58:71: W504 line break after binary operator

• In the file skimage/future/graph/rag.py:

Line 213:64: W504 line break after binary operator
Line 369:71: W504 line break after binary operator
Line 551:15: E127 continuation line over-indented for visual indent

• In the file skimage/future/graph/tests/test_rag.py:

Line 126:71: W504 line break after binary operator

[NelleV]

I'd like to postpone this backport a bit so that I can finish the release cleanly.

[jni]

@NelleV no problem at all, merge this at your leisure! Or just let us know. =)

[jni]

Anyway is there any issue at all with merging to the branch now that the tag is done?

[NelleV]

We're good to go (I'm a bit surprised that you would backport such significant changes).

[jni]

Well, as @jarrodmillman said, if someone installs scikit-image 0.16.1 and NetworkX 2.4 together, it will break. I consider that a big problem!