Add decorator that un-annotation-futures things.

This is a short-term way to check annotations are valid at runtime.
May revisit after 3.14 using annotationslib

See #6

_misc/_ensure_annotations.py

@@ -0,0 +1,54 @@
+#   Copyright 2020-present Michael Hall
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+
+import ast
+import inspect
+import sys
+from types import FunctionType
+
+_cycle_blocked = False
+
+
+def ensure_annotations[T: type | FunctionType](f: T) -> T:
+    """Ensure annotations are runtime valid.
+
+    Returns
+    -------
+    A modified version of the object passed in
+    without the annotations future in effect
+    """
+    # Prevents a cyclic compile issue
+    global _cycle_blocked  # noqa: PLW0603
+    if _cycle_blocked:
+        return f
+
+    new_ast = compile(
+        ast.parse(inspect.getsource(f)),
+        "<string>",
+        "exec",
+        flags=0,
+        dont_inherit=True,
+        optimize=1,
+    )
+
+    env = sys.modules[f.__module__].__dict__
+
+    _cycle_blocked = True
+    try:
+        exec(new_ast, env)  # noqa: S102
+    finally:
+        _cycle_blocked = False
+
+    return env[f.__name__]