Adding cython v0.20.2 in third-party.

third_party/cython/src/Cython/Includes/cpython/instance.pxd

+cdef extern from "Python.h":
+
+    ############################################################################
+    # 7.5.2 Instance Objects
+    ############################################################################
+
+    # PyTypeObject PyInstance_Type
+    #
+    # Type object for class instances.
+
+    int PyInstance_Check(object obj)
+    # Return true if obj is an instance.
+
+    object PyInstance_New(object cls, object arg, object kw)
+    # Return value: New reference.
+    # Create a new instance of a specific class. The parameters arg
+    # and kw are used as the positional and keyword parameters to the
+    # object's constructor.
+
+    object PyInstance_NewRaw(object cls, object dict)
+    # Return value: New reference.
+    # Create a new instance of a specific class without calling its
+    # constructor. class is the class of new object. The dict
+    # parameter will be used as the object's __dict__; if NULL, a new
+    # dictionary will be created for the instance.