some compatibility code

pdm/_types.py

@@ -1,5 +1,10 @@
 from typing import Dict, List, NamedTuple, Tuple, Union
 
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal  # noqa
+
 Source = Dict[str, Union[str, bool]]
 RequirementDict = Union[str, Dict[str, Union[bool, str]]]
 CandidateInfo = Tuple[List[str], str, str]

pdm/models/versions.py

@@ -1,6 +1,7 @@
 import re
-from typing import Any, Literal, Tuple, Union, overload
+from typing import Any, Tuple, Union, overload
 
+from pdm._types import Literal
 from pdm.exceptions import InvalidPyVersion
 
 VersionBit = Union[int, Literal["*"]]