Fix imports from conv.py (#4787)

blib2to3.conv cannot currently be imported.

Co-authored-by: cobalt <[email protected]>

Author: gordonmessmer

src/blib2to3/pgen2/conv.py

@@ -32,7 +32,7 @@
 import re
 
 # Local imports
-from pgen2 import grammar, token
+from blib2to3.pgen2 import grammar, token
 
 
 class Converter(grammar.Grammar):