Reject nauty version < 2.6 #29462
Description
We get the following failure on some systems, see e.g. https://github.com/kliem/sage-test-27122/runs/558808988:
File "src/sage/graphs/digraph_generators.py", line 600, in sage.graphs.digraph_generators.DiGraphGenerators.nauty_directg
Failed example:
dgs[0]
Exception raised:
Traceback (most recent call last):
File "/sage/local/lib/python3.7/site-packages/sage/doctest/forker.py", line 681, in _run
self.compile_and_execute(example, compiler, test.globs)
File "/sage/local/lib/python3.7/site-packages/sage/doctest/forker.py", line 1123, in compile_and_execute
exec(compiled, globs)
File "<doctest sage.graphs.digraph_generators.DiGraphGenerators.nauty_directg[3]>", line 1, in <module>
dgs[Integer(0)]
IndexError: list index out of range
**********************************************************************
File "src/sage/graphs/digraph_generators.py", line 602, in sage.graphs.digraph_generators.DiGraphGenerators.nauty_directg
Failed example:
dgs[0]._bit_vector()
Exception raised:
Traceback (most recent call last):
File "/sage/local/lib/python3.7/site-packages/sage/doctest/forker.py", line 681, in _run
self.compile_and_execute(example, compiler, test.globs)
File "/sage/local/lib/python3.7/site-packages/sage/doctest/forker.py", line 1123, in compile_and_execute
exec(compiled, globs)
File "<doctest sage.graphs.digraph_generators.DiGraphGenerators.nauty_directg[4]>", line 1, in <module>
dgs[Integer(0)]._bit_vector()
IndexError: list index out of range
The problem is that we use digraph6, a format which is not supported by nauty previous to 2.6 according to
http://users.cecs.anu.edu.au/~bdm/nauty/changes24-26.txt
CC: @mkoeppe @orlitzky @dimpase
Component: packages: standard
Keywords: nauty, digraph6
Author: Michael Orlitzky
Branch/Commit: 642fbc0
Reviewer: Jonathan Kliem
Issue created by migration from https://trac.sagemath.org/ticket/29462