py3: fix doctests failures of sage/combinat/words/word_*datatype*

[slabbe]

with 8.4.rc0, the python 3 failures in sage/combinat/words are:

----------------------------------------------------------------------
sage -t abstract_word.py  # 13 doctests failed
sage -t word_char.pyx  # 3 doctests failed
sage -t word_generators.py  # 23 doctests failed
sage -t word_infinite_datatypes.py  # 22 doctests failed
sage -t words.py  # 7 doctests failed
sage -t word.py  # 8 doctests failed
sage -t word_datatypes.pyx  # 1 doctest failed
sage -t suffix_trees.py  # 2 doctests failed
sage -t morphism.py  # 16 doctests failed
sage -t finite_word.py  # 73 doctests failed
sage -t paths.py  # 15 doctests failed
----------------------------------------------------------------------

often of the following form due to word_infinite_datatypes.py:

**********************************************************************
File "word_infinite_datatypes.py", line 1035, in sage.combinat.words.word_infinite_datatypes.WordDatatype_iter_with_caching.__getitem__
Failed example:
    w[::2]
Exception raised:
    Traceback (most recent call last):
      File "/home/slabbe/GitBox/sage3/local/lib/python3.6/site-packages/sage/doctest/forker.py", line 659, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/home/slabbe/GitBox/sage3/local/lib/python3.6/site-packages/sage/doctest/forker.py", line 1070, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.combinat.words.word_infinite_datatypes.WordDatatype_iter_with_caching.__getitem__[17]>", line 1, in <module>
        w[::Integer(2)]
      File "/home/slabbe/GitBox/sage3/local/lib/python3.6/site-packages/sage/combinat/words/word_infinite_datatypes.py", line 1161, in __getitem__
        return super(WordDatatype_iter_with_caching,self).__getitem__(key)
      File "/home/slabbe/GitBox/sage3/local/lib/python3.6/site-packages/sage/combinat/words/word_infinite_datatypes.py", line 874, in __getitem__
        data = itertools.islice(self, key.start, stop, key.step)
    ValueError: Step for islice() must be a positive integer or None.
**********************************************************************

We fix issues in word_infinite_datatypes.py and word_datatype.py and now with this ticket, there are a lot less failing doctests in sage/combinat/words:

----------------------------------------------------------------------
sage -t abstract_word.py  # 4 doctests failed
sage -t word_generators.py  # 2 doctests failed
sage -t word.py  # 1 doctest failed
sage -t morphism.py  # 9 doctests failed
sage -t finite_word.py  # 7 doctests failed
----------------------------------------------------------------------

[slabbe]

New commits:

​cf2f1c5

26473: py3 fix doctests of word_datatypes

[git]

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

​9c851be

26473: py3 fix doctests of combinat/words/words.py

[slabbe]

I am still working on it ...

[git]

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

​2a3e7c6

26473: py3 fix some doctests of combinat/words/word_generators.py

[git]

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

​b541ed5

26473: py3 fix some doctests of combinat/words/finite_word.py

[slabbe]

New commits:

​2a3e7c6	26473: py3 fix some doctests of combinat/words/word_generators.py
​b541ed5	26473: py3 fix some doctests of combinat/words/finite_word.py

[vklein]

Looks good to me.

Pyflakes complaints are unrelated with this ticket's modifications.

Thanks.

[embray]

This should be re-targeted for 8.5.