Use Python headers as system to avoid C++17 warnings from Python 2.7
Use Python headers as system to avoid C++17 warnings from Python 2.7

Python 2.7 headers use 'register', which is deprecated in C++17 and
leads to warnings:

python2.7/unicodeobject.h:534:5: warning:
  'register' storage class specifier is deprecated and incompatible with C++17

register PyObject *obj,     /* Object */
^~~~~~~~

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