Decode output from wal-e list backup

When running this script using Python3, the output is bytestring instead of string.
We explicitly decode it to ensure checks further down are ok.

The trigger for this patch is:

ERROR: unable to get some of WALE backup parameters: 'expanded_size_bytes'

patroni/scripts/wale_restore.py

@@ -73,7 +73,7 @@ class WALERestore(object):
             # base_00000001000000000000007F_00000040  2015-05-18T10:13:25.000Z
             # 20310671    00000001000000000000007F    00000040
             # 00000001000000000000007F    00000240
-            backup_strings = latest_backup.splitlines() if latest_backup else ()
+            backup_strings = latest_backup.decode('utf-8').splitlines() if latest_backup else ()
             if len(backup_strings) != 2:
                 return False