Fix encoding error (python 3)

[achauve]

BTW we now use MRQ successfully in production with python 3.5.2 at Serenytics :)
Thanks again @sylvinus for sharing this lib!

[sylvinus]

Yay! :)

[skuda]

This breaks using Python2, we had problems with that key for example:

In [20]: key = "XZ{Z\x0f\x96\xbcr\xea\xbdm\r"

In [21]: binascii.hexlify(key)
Out[21]: '585a7b5a0f96bc72eabd6d0d'

In [22]: binascii.hexlify(key.encode('utf-8') if isinstance(key, str) else key)
---------------------------------------------------------------------------
UnicodeDecodeError                        Traceback (most recent call last)
<ipython-input-22-25b650f21318> in <module>()
----> 1 binascii.hexlify(key.encode('utf-8') if isinstance(key, str) else key)

UnicodeDecodeError: 'ascii' codec can't decode byte 0x96 in position 5: ordinal not in range(128)

isinstance(key, str) means something different in Python3 and Python2, being unicode in Python 3 but bytes in Python2. maybe a check for sys.version_info < (3,) is needed?

[achauve]

Indeed the isinstance(key, str) is okay in python2 only if we use the str builtin from future:

from builtins import str