Create hello_world.py and unit test

.gitignore

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+__pycache__/

hello_world.py

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+def hello_world():
+    return "hello world"
+
+if __name__ == "__main__":
+    print(hello_world())

test_hello_world.py

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+import unittest
+from hello_world import hello_world
+
+class TestHelloWorld(unittest.TestCase):
+    def test_hello_world(self):
+        self.assertEqual(hello_world(), "hello world")
+
+if __name__ == '__main__':
+    unittest.main()