Fix logging errors on Python 3.9

* bdx/__init__.py (log.prettify_arg): Pass all 3 required arguments to
`format_exception'.

Author: mkcms

bdx/__init__.py

@@ -24,7 +24,9 @@ def prettify_arg(arg):
                 return os.path.relpath(arg)
 
             if isinstance(arg, Exception):
-                return "\n".join(tb.format_exception(arg))
+                return "\n".join(
+                    tb.format_exception(type(arg), arg, arg.__traceback__)
+                )
 
             return arg