Follow open() interface

[asmodehn]

For reference : https://docs.python.org/3/library/functions.html#open I would expect naively the API fo https://github.com/algrebe/python-tee/blob/master/tee/tee.py#L11 to be the same...

[algrebe]

My reference was https://docs.python.org/2/library/functions.html#open as I didn't intend to support python 3 at the time.

Will make sure to change it when I add support for python 3!