Remove some old python fallbacks

ForwardRef needs to explicitly call out recursive_guard in python 3.12.

python/lsst/pex/config/config.py

@@ -46,14 +46,9 @@
 import tempfile
 import warnings
 from collections.abc import Mapping
+from types import GenericAlias
 from typing import Any, ForwardRef, Generic, TypeVar, cast, overload
 
-try:
-    from types import GenericAlias
-except ImportError:
-    # cover python 3.8 usage
-    GenericAlias = type(Mapping[int, int])
-
 # if YAML is not available that's fine and we simply don't register
 # the yaml representer since we know it won't be used.
 try:
@@ -449,13 +444,8 @@ def _parseTypingArgs(
             _typ = ForwardRef(unpackedParams)
             # type ignore below because typeshed seems to be wrong. It
             # indicates there are only 2 args, as it was in python 3.8, but
-            # 3.9+ takes 3 args. Attempt in old style and new style to
-            # work with both.
-            try:
-                result = _typ._evaluate(globals(), locals(), set())  # type: ignore
-            except TypeError:
-                # python 3.8 path
-                result = _typ._evaluate(globals(), locals())
+            # 3.9+ takes 3 args.
+            result = _typ._evaluate(globals(), locals(), recursive_guard=set())  # type: ignore
             if result is None:
                 raise ValueError("Could not deduce type from input")
             unpackedParams = cast(type, result)

python/lsst/pex/config/dictField.py

@@ -263,13 +263,8 @@ def _parseTypingArgs(
                 _typ = ForwardRef(typ)
                 # type ignore below because typeshed seems to be wrong. It
                 # indicates there are only 2 args, as it was in python 3.8, but
-                # 3.9+ takes 3 args. Attempt in old style and new style to
-                # work with both.
-                try:
-                    result = _typ._evaluate(globals(), locals(), set())  # type: ignore
-                except TypeError:
-                    # python 3.8 path
-                    result = _typ._evaluate(globals(), locals())
+                # 3.9+ takes 3 args.
+                result = _typ._evaluate(globals(), locals(), recursive_guard=set())  # type: ignore
                 if result is None:
                     raise ValueError("Could not deduce type from input")
                 typ = cast(type, result)