Add python 3 compatibility

[FFY00]

With python 2 being discontinued in 9 months could you merge this to keep the project usable? Thanks in advance.

[linusg]

The provided changes are not enough, for example this needs to be changed as file doesn't exist in Python 3 anymore:

https://github.com/isaacl/meliae/blob/f4bda183bbd4f1285d8fd8c553e102d4325c1753/meliae/loader.py#L530-L533

Otherwise it'll die with:

NameError: name 'file' is not defined

Please don't let this project die @isaacl!