Backport PR #4132: Fixed name error bug in function safe_unicode in module py3compat.

IPython/utils/py3compat.py

@@ -60,12 +60,12 b' def safe_unicode(e):'
         pass
 
     try:
-        return py3compat.str_to_unicode(str(e))
+        return str_to_unicode(str(e))
     except UnicodeError:
         pass
 
     try:
-        return py3compat.str_to_unicode(repr(e))
+        return str_to_unicode(repr(e))
     except UnicodeError:
         pass