Explicitly call hmad constructor with md5 hash

[abbot]

kousu gave at heads up that:

     digestmod: A module supporting PEP 247.  *OR*
                A hashlib constructor returning a new hash object. *OR*
                A hash name suitable for hashlib.new().
                Defaults to hashlib.md5.
                Implicit default to hashlib.md5 is deprecated and will be
                removed in Python 3.6.

So instead of relying on MD5 being the default for hashlib, explicitly call hashlib constructor with md5 digest module/object/whatever.

[abbot]

Fixed in b44c7f2.