test: skip if os.unshare is not available

It was added in Python 3.12 which is very recent, so skip
gracefully if it is not available, like on the current
Debian stable or Ubuntu Jammy

https://docs.python.org/3/library/os.html#os.unshare

test/wrapper.py

@@ -32,6 +32,9 @@ def setup_test_namespace(data_dir):
     except PermissionError:
         print("Lacking permissions to set up test harness, skipping")
         sys.exit(77)
+    except AttributeError:
+        print("Python 3.12 is required for os.unshare(), skipping")
+        sys.exit(77)
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser()