Fix typing issue due to `try-except`

pysr/sr.py

@@ -13,9 +13,9 @@ from multiprocessing import cpu_count
 from pathlib import Path
 from typing import Callable, Dict, List, Optional, Tuple, Union
 
-try:
+if sys.version_info >= (3, 8):
     from typing import Literal
-except ImportError:
+else:
     from typing_extensions import Literal
 
 import numpy as np