No there is no call to PyType_GenericNew (or tp_new) here - this is just setting a variable (PWMType.tp_new) to the value PyType_GenericNew which happens to be a pointer to a function that will (if called) create a new instance. The important point here is that it is *NOT* being called, simply referred to and assigned to a variable.
Er no, the tp_new field is a function pointer which when called with appropriate parameters will instantiate a new object and return it, but here it is not being called. This code is to arrange that when it is called that functionality (required by the definition of tp_new) is provided by PyType_GenericNew. Without it the first attempt to instantiate a PWMType would result in a SEGV induced core dump.
In both C and C++ the syntax for 'call this_function' is this_function() possibly with arguments in the brackets but without the brackets there is no function being called. The code you are looking at is an assignment pure and simple with no more side effects than pi = 3;.