Fix Python 3 issue, use str instead of basestring

[michael-erasmus]

Hey @kennethreitz!

Just a small PR to fix an issue I got trying to use this with Python3.

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/opt/conda/lib/python3.5/site-packages/dynamo.py", line 48, in __setitem__
    if isinstance(key, (basestring, Number)):
NameError: name 'basestring' is not defined

This was pretty easy to fix, just forked the repo and ran:

2to3 -wn dynamo.py