Reading files is broken in Python 3.9

[mfncooper]

At least on Python 3.9 (I have not tested other versions), the file read loop here:

https://github.com/mumbly/PyUtils/blob/master/FileDupeFinder.py#L54

never finishes. Because the file is opened in binary mode, read() returns b'' and not '' at the end of the file. Thus the sentinel never matches, and the loop goes on forever.