Fix Transformation type protocol

Adding ureg to the argument list for Transformation Protocol

Closes #1801

CHANGES

@@ -4,6 +4,8 @@ Pint Changelog
 0.23 (unreleased)
 -----------------
 
+- Fixed Transformation type protocol.
+  (PR #1805)
 - Documented to_preferred and created added an autoautoconvert_to_preferred registry option.
   (PR #1803)
 

pint/facets/context/objects.py

@@ -10,17 +10,22 @@
 
 import weakref
 from collections import ChainMap, defaultdict
-from typing import Any, Callable, Protocol, Generic, Optional
+from typing import Any, Callable, Protocol, Generic, Optional, TYPE_CHECKING
 from collections.abc import Iterable
 
 from ...facets.plain import UnitDefinition, PlainQuantity, PlainUnit, MagnitudeT
 from ...util import UnitsContainer, to_units_container
 from .definitions import ContextDefinition
 from ..._typing import Magnitude
 
+if TYPE_CHECKING:
+    from ...registry import UnitRegistry
+
 
 class Transformation(Protocol):
-    def __call__(self, value: Magnitude, **kwargs: Any) -> Magnitude:
+    def __call__(
+        self, ureg: UnitRegistry, value: Magnitude, **kwargs: Any
+    ) -> Magnitude:
         ...