Python 3 compatability fix

In Python 3 string literals are str, and not bytes, so `DictAttribute.__set___` would pass the wrong type of argument to the C functino `drmaa_set_vector_attribute`. This simple change encodes strings to bytes with the system default encoding. Since you can called `.encode()` on byte strings in Python 2.x without side-effect, this simple change should work with both.

drmaa/helpers.py

@@ -133,7 +133,7 @@ class DictAttribute(object):
     def __init__(self, name):
         self.name = name
     def __set__(self, instance, value):
-        v = [ "%s=%s" % (k, v) for (k, v) in value.items() ]
+        v = [ "%s=%s".encode() % (k, v) for (k, v) in value.items() ]
         c(drmaa_set_vector_attribute, instance, self.name, string_vector(v))
     def __get__(self, instance, _):
         x = [ i.split('=', 1) for i in list(vector_attribute_iterator(