Check against six.string_types instead of str for python 2 compatibility

[aubustou]

If you named your columns with Unicode in Python 2, it obviously won't work...
In order to keep Python 2 and 3 compatibilities, better to use six.string_types.

[coveralls]

Coverage increased (+0.02%) to 97.323% when pulling 59711f4 on aubustou:master into a361c52 on graphql-python:master.

[jnak]

Makes sense.

It looks like you have a minor lint issue. You should be able to fix it automatically by running tox -e pre-commit (see how to set up your local dev env)

Also would you mind adding or updating a test case to prevent regressions in the future?

Let me know if you run into any issue.
Thanks

[jnak]

You need to run tox -e pre-commit before pushing your changes. It will re-organize automatically your imports so your tests don't fail.

[aubustou]

Once i manage to install that damned Python 3.7 on my install, I'll fix it.

[jnak]

FYI we usually require to add test cases to PR in order to prevent future regressions. But since it's a python 2.7 compatibility fix and Python 2.7 end-of-life is in 4 months, I'm fine merging as is.

Thank you for pushing this PR through!