Qualify use of "__builtin__.Exception" class.

It is possible that the module we're wrapping defines an Exception
class. This will confuse code that uses an unqualified "Exception"
class (e.g. "try: ... except Exception") since it now won't match
the Python builtin Exception. Fix this by explicitly using
the class from the __builtin__ module ("builtins" in Python 3).

Examples/test-suite/python/exception_classname_runme.py

@@ -0,0 +1,4 @@
+import exception_classname
+
+a = exception_classname.Exception()
+assert a.testfunc() == 42