Install `typing_extensions` for compat with older Python

pysr/sr.py

@@ -11,7 +11,12 @@ from datetime import datetime
 from io import StringIO
 from multiprocessing import cpu_count
 from pathlib import Path
-from typing import Callable, Dict, List, Literal, Optional, Tuple, Union
+from typing import Callable, Dict, List, Optional, Tuple, Union
+
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
 
 import numpy as np
 import pandas as pd

requirements.txt

@@ -5,3 +5,4 @@ scikit_learn>=1.0.0
 julia>=0.6.0
 click>=7.0.0
 setuptools>=50.0.0
+typing_extensions>=4.0.0,<5.0.0