allow graphView to be pickled and unpickled

[TeunHuijben]

Previously, (un)pickling the graphView gave problems, because the _edge_from_{from/to}_root bidict got duplicated and was no longer coupled.

In this PR, we add getstate/setstate methods to the GraphView and mappedGraphMixin to make the bidict picklable.

[TeunHuijben]

Two questions for @JoOkuma:

• is it okay that the tests only run for GraphView backend, and not for the standard graphs? Shouldn't we be able to pickle any graph?
• the problem with pickling the other graphs, is that the graph_backend fixture adds the _add_node_with_index methodsto the class, which gives problems during unpickling

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.37%. Comparing base (f14aaa4) to head (c1268b4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   88.40%   88.37%   -0.04%     
==========================================
  Files          55       55              
  Lines        3873     3940      +67     
  Branches      674      690      +16     
==========================================
+ Hits         3424     3482      +58     
- Misses        267      276       +9     
  Partials      182      182              

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[JoOkuma]

Hi @TeunHuijben, I did my review as PR in #233 , can you check if you agree with them?