Fix Python 3 compatability issue with setting DictAttributes

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.