Fix mypy errors in test_symbols and server SZL list

- test_symbols.py: use isinstance check for ValueType union
- server/__init__.py: avoid bytes/bytearray type mismatch in SZL list

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gijzelaerr

snap7/server/__init__.py

@@ -1704,10 +1704,10 @@ def _get_szl_data(self, szl_id: int, szl_index: int) -> Optional[bytes]:
         elif szl_id == 0x0000:
             # Return list of available SZL IDs
             available_ids = [0x0000, 0x0011, 0x001C, 0x0131, 0x0232]
-            data = b""
+            szl_bytes = b""
             for id_val in available_ids:
-                data += struct.pack(">H", id_val)
-            return data
+                szl_bytes += struct.pack(">H", id_val)
+            return szl_bytes
 
         return None
 

tests/test_symbols.py

@@ -407,7 +407,9 @@ def test_read_many_single_tag(self) -> None:
 
         table = SymbolTable({"Temp": {"db": 1, "offset": 0, "type": "REAL"}})
         values = table.read_many(client, ["Temp"])
-        assert abs(values["Temp"] - 42.0) < 0.01
+        temp = values["Temp"]
+        assert isinstance(temp, float)
+        assert abs(temp - 42.0) < 0.01
 
     def test_read_many_empty(self) -> None:
         client = _make_client()