Consider removing python2.6 from AppVeyor

[daa]

Now tests at AppVeyour partially fail because pytest requires Python >= 2.7. May be it's time to deprecate 2.6 support or to not test it at least?

[hugovk]

Drop it entirely (from Travis CI etc.).

Python 2.6 has been EOL for four years and no longer receives security updates. Many other important Python libraries are also dropping it, and it's little used.

https://en.wikipedia.org/wiki/CPython#Version_history

Here's the pip installs for marisa-trie from PyPI for the last month (via pypinfo --percent --pip marisa-trie pyversion), showing virtually no usage.

python_version	percent	download_count
2.7	59.3%	3,048
3.5	20.9%	1,074
3.6	14.3%	734
3.4	5.4%	279
2.6	0.1%	4