pythongh-104050: Argument clinic: Complete get_destination_buffer annotations (python#107293)

Author: AlexWaygood

Tools/clinic/clinic.py

@@ -2172,7 +2172,7 @@ def __init__(
             'impl_definition': d('block'),
         }
 
-        DestBufferType = dict[str, Callable[..., Any]]
+        DestBufferType = dict[str, _TextAccumulator]
         DestBufferList = list[DestBufferType]
 
         self.destination_buffers_stack: DestBufferList = []
@@ -2226,7 +2226,7 @@ def get_destination_buffer(
             self,
             name: str,
             item: int = 0
-    ):
+    ) -> _TextAccumulator:
         d = self.get_destination(name)
         return d.buffers[item]