Reject nauty version < 2.6

[kliem]

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

[orlitzky]

Commit: 642fbc0

[orlitzky]

New commits:

642fbc0

Trac #29462: check for the "converseg" nauty utility.

[orlitzky]

Branch: u/mjo/ticket/29462

[orlitzky]

Author: Michael Orlitzky

[kliem]

Reviewer: Jonathan Kliem

[kliem]

comment:3

I would say it is good to go. Does anyone think this needs testing?

I checked that indeed converseg doesn't show up in version 2.5b9 but it is there in version 2.6.

[kliem]

comment:4

I just started a test #29462, #29442, #29440, #29272, #29445.

https://github.com/kliem/sage-test-27122/actions/runs/70559675

I'll put it on positive review, if this works as desired.

[kliem]

comment:5

Ok. Works.

E.g. nauty is being rejected on ubuntu-xenial, so that the doctest passes. But it is accepted by ubuntu-bionic.

[vbraun]

Changed branch from u/mjo/ticket/29462 to 642fbc0