Fix new variable name using

Author: Alexpux

Lib/importlib/_bootstrap_external.py

@@ -43,7 +43,7 @@
 else:
     path_separators = ['/']
 
-if sys.use_alt_sep:
+if sys._use_alt_sep:
     path_separators = path_separators[::-1]
 
 # Assumption made in _path_join()