Change shebang to point to python2

I'd assume that `python` will gradually change from pointing to Python 2 to Python 3 while Python 3 will become more widespread.

`python2` on the other hand is avialable on most systems to enable legacy Python 2 scripts to still run. Xbev is incompatible with Python 3, so I would suggest changing the shebang to point to Python 2.