python3 support

[rathann]

Building and using the python extension with python3 fails. Apart from fixing detection in configure (macros/python.m4), it's also necessary to port (py_ext/ming.py) to python3 (2to3 works fairly well, but doesn't fix whitespace inconsistencies). However, that's still not enough. Apparently, the SWIG bindings use python2-only APIs like PyFile_Check and PyFile_AsFile, which are not available in python3. See https://docs.python.org/3/c-api/file.html and https://stackoverflow.com/questions/16130268/python-3-replacement-for-pyfile-asfile .

[cbehan]

I don't use the API but it would seem that a change I just made to the SWIG bindings has a chance of working. We can find out if it's worthwhile to submit if you want to give it a try.