Improve error message if pytest.raises is used wrongly

If the type is not checked then an incomprehensible error will occur
later.  This enforces the type and raies the same exception/msg as
CPython does in that case.

Docstring unmodified, just re-justified for pep8 compat.

testing/python/raises.py

@@ -62,3 +62,10 @@ class TestRaises:
             '*3 passed*',
         ])
 
+    def test_noclass(self):
+        with pytest.raises(TypeError):
+            pytest.raises('wrong', lambda: None)
+
+    def test_tuple(self):
+        with pytest.raises((KeyError, ValueError)):
+            raise KeyError('oops')