bpo-32492: 1.6x speed up in namedtuple attribute access using C fast-path (#10495)

bpo-32492: 1.6x speed up in namedtuple attribute access using C fast-path (#10495)

* bpo-32492: 2.5x speed up in namedtuple attribute access using C fast path

* Add News entry

* fixup! bpo-32492: 2.5x speed up in namedtuple attribute access using C fast path

* Check for tuple in the __get__ of the new descriptor and don't cache the descriptor itself

* Don't inherit from property. Implement GC methods to handle __doc__

* Add a test for the docstring substitution in descriptors

* Update NEWS entry to reflect time against 3.7 branch

* Simplify implementation with argument clinic, better error messages, only __new__

* Use positional-only parameters for the __new__

* Use PyTuple_GET_SIZE and PyTuple_GET_ITEM to tighter the implementation of tuplegetterdescr_get

* Implement __set__ to make tuplegetter a data descriptor

* Use Py_INCREF now that we inline PyTuple_GetItem

* Apply the valid_index() function, saving one test

* Move Py_None test out of the critical path.