Fix encoding conversion in CFF

Was broken with Python 3 and Salsa-Regular.otf for example.
diff --git a/Lib/fontTools/cffLib.py b/Lib/fontTools/cffLib.py
index e3b00c9..049ff97 100644
--- a/Lib/fontTools/cffLib.py
+++ b/Lib/fontTools/cffLib.py
@@ -1610,7 +1610,7 @@
 		if file is None:
 			strings = []
 		else:
-			strings = [tostr(s) for s in Index(file)]
+			strings = [tostr(s, encoding="latin1") for s in Index(file)]
 		self.strings = strings
 	
 	def getCompiler(self):