only apply Literal type checking for 3.8 and higher

sqlalchemy-stubs/dialects/postgresql/base.pyi

@@ -1,9 +1,9 @@
 from ... import schema
 from ...engine import default, reflection
 from ...sql import compiler, expression, sqltypes, type_api
-from typing import Any, Optional, Set, Type, Text, Pattern, Dict, TypeVar, overload, Literal
+from typing import Any, Optional, Set, Type, Text, Pattern, Dict, TypeVar
 from datetime import timedelta
-import uuid
+import sys
 
 from sqlalchemy.types import INTEGER as INTEGER, BIGINT as BIGINT, SMALLINT as SMALLINT, VARCHAR as VARCHAR, \
     CHAR as CHAR, TEXT as TEXT, FLOAT as FLOAT, NUMERIC as NUMERIC, \
@@ -72,10 +72,15 @@ _T = TypeVar("_T")
 class UUID(sqltypes.TypeEngine[_T]):
     __visit_name__: str = ...
     as_uuid: bool = ...
-    @overload
-    def __new__(self, as_uuid: Literal[True]) -> UUID[sqltypes.TypeEngine[uuid.UUID]]: ...
-    @overload
-    def __new__(self, as_uuid: Literal[False, None]) -> UUID[sqltypes.TypeEngine[str]]: ...
+    if sys.version_info() >= (3,8):
+        # Literal type is only supported since 3.8
+        from typing import Literal, overload
+        import uuid
+        @overload
+        def __new__(self, as_uuid: Literal[True]) -> UUID[sqltypes.TypeEngine[uuid.UUID]]: ...
+        @overload
+        def __new__(self, as_uuid: Literal[False, None]) -> UUID[sqltypes.TypeEngine[str]]: ...
+
     def __new__(self) -> UUID[sqltypes.TypeEngine[str]]: ...
     def bind_processor(self, dialect: Any): ...
     def result_processor(self, dialect: Any, coltype: Any): ...