Adding cython v0.20.2 in third-party.

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

+cdef extern from "Python.h":
+
+    ############################################################################
+    # 7.2.3
+    ############################################################################
+    # PyFloatObject
+    #
+    # This subtype of PyObject represents a Python floating point object.
+
+    # PyTypeObject PyFloat_Type
+    #
+    # This instance of PyTypeObject represents the Python floating
+    # point type. This is the same object as float and
+    # types.FloatType.
+
+    bint PyFloat_Check(object p)
+    # Return true if its argument is a PyFloatObject or a subtype of
+    # PyFloatObject.
+
+    bint PyFloat_CheckExact(object p)
+    # Return true if its argument is a PyFloatObject, but not a
+    # subtype of PyFloatObject.
+
+    object PyFloat_FromString(object str, char **pend)
+    # Return value: New reference.
+    # Create a PyFloatObject object based on the string value in str,
+    # or NULL on failure. The pend argument is ignored. It remains
+    # only for backward compatibility.
+
+    object PyFloat_FromDouble(double v)
+    # Return value: New reference.
+    # Create a PyFloatObject object from v, or NULL on failure.
+
+    double PyFloat_AsDouble(object pyfloat) except? -1
+    # Return a C double representation of the contents of pyfloat.
+
+    double PyFloat_AS_DOUBLE(object pyfloat)
+    # Return a C double representation of the contents of pyfloat, but
+    # without error checking.