[Bugfix] fix type hint for py 3.8 (#4036)

vllm/executor/executor_base.py

 from abc import ABC, abstractmethod
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Tuple
 
 from vllm.config import (CacheConfig, DeviceConfig, LoRAConfig, ModelConfig,
                          ParallelConfig, SchedulerConfig, SpeculativeConfig,
@@ -31,7 +31,7 @@ class ExecutorBase(ABC):
         raise NotImplementedError
 
     @abstractmethod
-    def determine_num_available_blocks(self) -> tuple[int, int]:
+    def determine_num_available_blocks(self) -> Tuple[int, int]:
         """Determine the number of available blocks for the GPU KV cache and
         swappable CPU KV cache.