py3: fix doctest in generic_graph (part 5)

[dcoudert]

Fix doctests in

• lex_BFS: mark some doctests as py2 or py3
• _color_by_label: mark some doctests as py2 or py3
• layout_extend_randomly: mark a doctest as # random and add another test

[dcoudert]

New commits:

​df37258

trac #27183: other doctests

[gh-jfraymond]

Hello,

when running the tests on generic_graph.py I got:

**********************************************************************
File "src/sage/graphs/generic_graph.py", line 18636, in sage.graphs.generic_graph.GenericGraph.layout_extend_randomly
Failed example:
    H.layout_extend_randomly({('0', 0): (0, 0), ('1', 1): (1, 1)})  # py3
Expected:
    {('0', 0): (0, 0),
     ('0', 1): [0.1114..., 0.514...],
     ('1', 0): [0.0446..., 0.332...],
     ('1', 1): (1, 1)}
Got:
    {('0', 0): (0, 0),
     ('0', 1): [0.04468968524815642, 0.332490606442413],
     ('1', 0): [0.111439293741037, 0.5143475134191677],
     ('1', 1): (1, 1)}
**********************************************************************

It is not surprising as the results are supposed to be random. Is there a way to specify this in the doctests? (or to set the random seed to a fixed value before running the tests?)

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​d0dc10a	trac #27183: Merged with 8.7.beta3
​78ba5cf	trac #27183: better doctest in layout_extend_randomly

[dcoudert]

I changed the doctest to mark the output as random and added a test on the bounding box.

[gh-jfraymond]

Thank you.