Fix the "SyntaxError: Non-ASCII character '\xe2'"

[vbraun]

There is a non-ascii character that messes up doctests for some:

File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/devel/sage/sage/graphs/generators/smallgraphs.py", line 2870:
    sage: graphs.TutteCoxeterGraph(embedding=3)
Expected:
    Traceback (most recent call last):
    ...
    ValueError: The value of embedding must be 1 or 2.
Got:
    Traceback (most recent call last):
      File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/bin/ncadoctest.py", line 1231, in run_one_test
        self.run_one_example(test, example, filename, compileflags)
      File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/bin/sagedoctest.py", line 38, in run_one_example
        OrigDocTestRunner.run_one_example(self, test, example, filename, compileflags)
      File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/bin/ncadoctest.py", line 1172, in run_one_example
        compileflags, 1) in test.globs
      File "<doctest __main__.example_42[9]>", line 1, in <module>
        graphs.TutteCoxeterGraph(embedding=Integer(3))###line 2870:
    sage: graphs.TutteCoxeterGraph(embedding=3)
      File "lazy_import.pyx", line 212, in sage.misc.lazy_import.LazyImport.__getattr__ (sage/misc/lazy_import.c:1865)
      File "lazy_import.pyx", line 148, in sage.misc.lazy_import.LazyImport._get_object (sage/misc/lazy_import.c:1268)
      File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/lib/python/site-packages/sage/graphs/graph_generators.py", line 291, in <module>
        class GraphGenerators():
      File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/lib/python/site-packages/sage/graphs/graph_generators.py", line 938, in GraphGenerators
        import sage.graphs.generators.smallgraphs
      File "/mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/lib/python/site-packages/sage/graphs/generators/smallgraphs.py", line 1132
    SyntaxError: Non-ASCII character '\xc3' in file /mnt/storage2TB/patchbot/Sage/sage-5.6.beta3/local/lib/python/site-packages/sage/graphs/generators/smallgraphs.py on line 1133, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details

[vbraun]

This comes from here btw:

def DurerGraph():
    r"""
    Returns the Dürer graph.

I propose we just switch to UTF-8 encoding and accept the fact that there are non-English names.

[vbraun]

Initial patch

[ncohen]

Duplicate of #13936

I remove the author's field because of Jeroen's email from this morning :-P

Nathann