Add cdef class example

Author: gjbex

source-code/cython/Classes/Makefile

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+VERSION = cpython-38-x86_64-linux-gnu
+PRIMES_LIB = primes_cython.$(VERSION).so
+
+all: $(PRIMES_LIB)
+
+$(PRIMES_LIB): points.pyx
+	python setup.py build_ext --inplace
+
+clean:
+	python setup.py clean
+	rm -f primes_cython.c $(PRIMES_LIB)

source-code/cython/Classes/README.md

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+# Classes
+
+Illustration of using Cython extension types (aka cdef classes).
+
+## What is it?
+
+1. `points.pyx`: implementation of a cdef class, and a Python
+   child class thereof.
+1. `driver.py`: Python script that uses both classes.
+1. `setup.py`: Python installation file to build the Cython
+   extension.
+1. `Makefile`: make file to build the extension.

source-code/cython/Classes/driver.py

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+#!/usr/bin/env python
+
+from points import Point, ColoredPoint
+
+p = Point(1.0, -2.0)
+print(p)
+print(f'point = {p.x}, {p.y}')
+print(p.distance())
+
+p1 = ColoredPoint(1.0, -2.0, 'blue')
+print(p1.color)