PythonGlue: initialize flight object on __new__() instead of __init__()

The __new__() method can be called only once and cannot be overwritten
which is therefore the safer place to initialize the flight object.
PythonGlue: incref on py_input_data

PyString_AsString returns only a reference to the string, so we have to
increase the reference counter on it to prevent the gc to destroy it.