Use io.StringIO in place of StringIO. As per recommendation from Pyth…

[mlsamuelson]

As per recommendation from Python 3.0 changelog (https://docs.python.org/3.0/whatsnew/3.0.html) use io.StringIO in place of StringIO.

io.StringIO is available in both Python 2.7 and 3.6.

Impetus for this change is that I started receiving the following error after upgrading from Flask-Ask 0.9.6.

  File "/var/task/flask_ask/core.py", line 6, in <module>
  from StringIO import StringIO
ModuleNotFoundError: No module named 'StringIO'

[mlsamuelson]

Looks like there's already two similar pull requests for this issue. Apologies for the extra noise.

[samalba]

+1 for merging, master is broken with python3...