fix SyntaxError on recent Python3

Author: a-detiste

dbf/_index.py

@@ -9,9 +9,9 @@ def __init__(self, dbf):
         filename += '.pdx'
         if not os.path.exists(filename):
             self.index_file = open(filename, 'r+b')
-            self.index_file.write('\xea\xaf\x37\xbf'    # signature
-                '\x00'*8                                # two non-existant lists
-                '\x00'*500)                             # and no indices
+            self.index_file.write(b'\xea\xaf\x37\xbf' +    # signature
+                                  b'\x00'*8           +    # two non-existant lists
+                                  b'\x00'*500)             # and no indices
             return
         index_file = self.index_file = open(filename, 'r+b')
         header = index_file.read(512)