Mention the ISC license for compat.py not only in the file itself

The file was taken & modified from
https://github.com/kennethreitz/requests which was licensed under the
ISC license until
https://github.com/kennethreitz/requests/commit/5c1bc201c46297d39d591e5d30e0e65a989bd22c

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>

This is the pull request for issue #110 to make sure this is ok.