more test for raise ... from ...

tests/run/test_raisefrom.pyx

+import unittest
+# adapted from pyregr
+class TestCause(unittest.TestCase):
+    def test_invalid_cause(self):
+        try:
+            raise IndexError from 5
+        except TypeError as e:
+            self.assertIn("exception cause", str(e))
+        else:
+            self.fail("No exception raised")
+
+    def test_class_cause(self):
+        try:
+            raise IndexError from KeyError
+        except IndexError as e:
+            self.assertIsInstance(e.__cause__, KeyError)
+        else:
+            self.fail("No exception raised")
+
+    def test_instance_cause(self):
+        cause = KeyError()
+        try:
+            raise IndexError from cause
+        except IndexError as e:
+            self.assertTrue(e.__cause__ is cause)
+        else:
+            self.fail("No exception raised")
+
+    def test_erroneous_cause(self):
+        class MyException(Exception):
+            def __init__(self):
+                raise RuntimeError()
+
+        try:
+            raise IndexError from MyException
+        except RuntimeError:
+            pass
+        else:
+            self.fail("No exception raised")