Fix 3.9 issue

Author: CoolCat467

src/sanescansrv/htmlgen.py

@@ -22,11 +22,13 @@
 __author__ = "CoolCat467"
 __license__ = "GNU General Public License Version 3"
 
-from typing import TYPE_CHECKING, TypeAlias, Union
+from typing import TYPE_CHECKING, Union
 
 if TYPE_CHECKING:  # pragma: nocover
     from collections.abc import Generator, Iterable, Mapping
 
+    from typing_extensions import TypeAlias
+
 
 def indent(level: int, text: str) -> str:
     """Indent text by level of spaces."""