fix: faithful is a class-var (#47)

Signed-off-by: nstarman <[email protected]>

src/jax_quantity/_core.py

@@ -6,7 +6,7 @@
 import operator
 from collections.abc import Callable, Sequence
 from dataclasses import replace
-from typing import TYPE_CHECKING, Any, TypeVar, final
+from typing import TYPE_CHECKING, Any, ClassVar, TypeVar, final
 
 import equinox as eqx
 import jax
@@ -79,7 +79,7 @@ def __check_init__(self) -> None:
     # ---------------------------------------------------------------
     # Plum stuff
 
-    __faithful__: bool = True
+    __faithful__: ClassVar[bool] = True
     """Tells :mod:`plum` that this type can be cached more efficiently."""
 
     @classmethod