Here is a slightly better solution. this overrides the _open method that is used. setting the umask before creating then returning it back to what it was.
class GroupWriteRotatingFileHandler(logging.handlers.RotatingFileHandler):
def _open(self):
prevumask=os.umask(0o002)
#os.fdopen(os.open('/path/to/file', os.O_WRONLY, 0600))
rtv=logging.handlers.RotatingFileHandler._open(self)
os.umask(prevumask)
return rtv