fix: zfs send uses str(path)

zfs-backup.py

@@ -257,7 +257,7 @@ class ZFS:
             raise ValueError(SAME_DATASET)
         print(f"  {old_snap} -> {new_snap}")
         sender = Popen(
-            [config.ZFS, "send", "-R", "-I", old_snap, new_snap], stdout=PIPE
+            [config.ZFS, "send", "-R", "-I", str(old_snap), str(new_snap)], stdout=PIPE
         )
         return sender