Merge dev_urandom_python() and dev_urandom_noraise() functions to reduce code
duplication.

Modify py_getrandom() to not call PyErr_CheckSignals() if raise is zero.
_PyRandom_Init() is called very early in the Python initialization, so it's
safer to not call PyErr_CheckSignals().

* Add pyurandom() helper function to factorize the code
* don't call Py_FatalError() in helper functions, but only in _PyRandom_Init()
  if pyurandom() failed, to uniformize the code

_is_data_descriptor().

_is_data_descriptor().

by increasing the number of iterations.  That appears sufficient to
expose the ref count problem fixed in this issue.
Patch suggested by Xiang Zhang

- be clear builtin traceback display was also updated
- show example output in What's New
- fix versionadded markup

to ref count problem introduced in code for Issue #27038 in 3.6.0a3.
Patch by Xiang Zhang.

directory and sysconfigdata file name.  This is a workaround for 3.6.0a4
pending resolution of other #23968 items.

Patch by Lisa Roach. See also PEP 628.