changed unraisable exception type to base exception

- print exception type to screen to match cpython behaviour
- included sample script to illustrate behaviour at sys.exit()

examples/atexit_example.py

@@ -0,0 +1,7 @@
+import atexit
+import sys
+
+def myexit():
+    sys.exit(2)
+
+atexit.register(myexit)

vm/src/stdlib/sys.rs

@@ -549,12 +549,12 @@ mod sys {
         }
         assert!(unraisable
             .exc_type
-            .fast_issubclass(vm.ctx.exceptions.exception_type));
+            .fast_issubclass(vm.ctx.exceptions.base_exception_type));
 
         // TODO: print module name and qualname
 
         if !vm.is_none(&unraisable.exc_value) {
-            write!(stderr, ": ");
+            write!(stderr, "{}: ", unraisable.exc_type);
             if let Ok(str) = unraisable.exc_value.str(vm) {
                 write!(stderr, "{}", str.as_str());
             } else {