remove unused import (pythonGH-5040) (python#5042)

(cherry picked from commit e325608)

Author: miss-islington

Lib/lib2to3/patcomp.py

@@ -12,7 +12,6 @@
 
 # Python imports
 import io
-import os
 
 # Fairly local imports
 from .pgen2 import driver, literals, token, tokenize, parse, grammar