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:

meliae/meliae/loader.py

Lines 530 to 533 in f4bda18

	if isinstance(source, file):
	input_size = os.fstat(source.fileno()).st_size
	else:
	input_size = 0

Otherwise it'll die with:

NameError: name 'file' is not defined

Please don't let this project die @isaacl!