Error when restoring compressed WALs with Python 2

Detects when barman-cloud-wal-restore is attempting to restore a
compressed WAL on python 2.x and returns an error message telling
the user to upgrade to a supported python 3.x.

See issue #325

barman/clients/cloud_walrestore.py

@@ -18,11 +18,13 @@
 
 import logging
 import os
+import sys
 from contextlib import closing
 
 import barman
 from barman.cloud import configure_logging
 from barman.cloud_providers import get_cloud_interface
+from barman.exceptions import BarmanException
 from barman.utils import force_str
 from barman.xlog import hash_dir, is_any_xlog_file, is_backup_file
 
@@ -232,6 +234,12 @@ def download_wal(self, wal_name, wal_dest):
             )
             raise SystemExit(1)
 
+        if compression and sys.version_info < (3, 0, 0):
+            raise BarmanException(
+                "Compressed WALs cannot be restored with Python 2.x - "
+                "please upgrade to a supported version of Python 3"
+            )
+
         # Download the file
         logging.debug(
             "Downloading %s to %s (%s)",