nixos-render-docs: add Freezable class

for most of our data classes we can use dataclasses.dataclass with
frozen=True or even plain named tuples. the TOC structure we'll need to
generate proper navigation links is most easily represented and used as
a cyclic structure though, and for that we can use neither. if we want
to make the TOC structures immutable (which seems like a good idea)
we'll need a hack of *some* kind, and this hack seems like the least intrusive.

pkgs/tools/nix/nixos-render-docs/src/nixos_render_docs/utils.py

@@ -0,0 +1,21 @@
+from typing import Any
+
+_frozen_classes: dict[type, type] = {}
+
+# make a derived class freezable (ie, disallow modifications).
+# we do this by changing the class of an instance at runtime when freeze()
+# is called, providing a derived class that is exactly the same except
+# for a __setattr__ that raises an error when called. this beats having
+# a field for frozenness and an unconditional __setattr__ that checks this
+# field because it does not insert anything into the class dict.
+class Freezeable:
+    def freeze(self) -> None:
+        cls = type(self)
+        if not (frozen := _frozen_classes.get(cls)):
+            def __setattr__(instance: Any, n: str, v: Any) -> None:
+                raise TypeError(f'{cls.__name__} is frozen')
+            frozen = type(cls.__name__, (cls,), {
+                '__setattr__': __setattr__,
+            })
+            _frozen_classes[cls] = frozen
+        self.__class__ = frozen