tests/basics: Add tests for type-checking subclassed exc instances.

tests/basics/subclass_native_exc_new.py

+# test subclassing exceptions and providing __new__
+
+
+class Dummy(BaseException):
+    pass
+
+
+class GoodException(BaseException):
+    def __new__(cls, *args, **kwargs):
+        print("GoodException __new__")
+        return Dummy(*args, **kwargs)
+
+
+class BadException(BaseException):
+    def __new__(cls, *args, **kwargs):
+        print("BadException __new__")
+        return 1
+
+
+try:
+    raise GoodException("good message")
+except BaseException as good:
+    print(type(good), good.args[0])
+
+try:
+    raise BadException("bad message")
+except Exception as bad:
+    # Should be TypeError 'exceptions must derive from BaseException'
+    print(type(bad), bad.args[0])
+
+try:
+
+    def gen():
+        yield
+
+    gen().throw(BadException)
+except Exception as genbad:
+    # Should be TypeError 'exceptions must derive from BaseException'
+    print(type(genbad), genbad.args[0])

tests/basics/subclass_native_exc_new.py.exp

+GoodException __new__
+<class 'Dummy'> good message
+BadException __new__
+<class 'TypeError'> exceptions must derive from BaseException
+BadException __new__
+<class 'TypeError'> exceptions must derive from BaseException