Fix an oversight in r83294.  unquote() should reject bytes.  Issue #9301.
diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
index a9fa26a..133b9d9 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -338,7 +338,7 @@
 
     unquote('abc%20def') -> 'abc def'.
     """
-    if string in (b'', ''):
+    if string == '':
         return string
     res = string.split('%')
     if len(res) == 1: