Handle typing limitations on Python 3.9

src/idaes_flowsheet_processor/_testing/plugins.py

@@ -1,11 +1,16 @@
+from __future__ import annotations  # allows using A | B for Python < 3.10
 import importlib
 from collections.abc import Iterable
 from collections.abc import Mapping
 from importlib.metadata import entry_points
 from importlib.metadata import EntryPoint
 from pathlib import Path
 from types import ModuleType
-from typing import TypeAlias
+
+try:
+    from typing import TypeAlias
+except ImportError:
+    TypeAlias = type
 
 import pytest