Minor optimizations to be made

The code has now been converted to Python 3 and slightly optimized. We can improve on this a little bit more by using `os.path(...)` here: https://github.com/exponential-decay/pronom-xml-export/blob/f70627de319ca37f37722319d7f47c903f8e9ecb/pronom-xml-export.py#L38-L44 and there are some areas where string concatenation can be improved. Shouldn't be too far away though. 