Clarify dtype type in ColumnBuffers elements (#87)

Co-authored-by: Matthew Barber <[email protected]>

protocol/dataframe_protocol.py

@@ -58,6 +58,9 @@ class DtypeKind(enum.IntEnum):
     CATEGORICAL = 23
 
 
+Dtype = Tuple[DtypeKind, int, str, str]  # see Column.dtype
+
+
 class ColumnNullType(enum.IntEnum):
     """
     Integer enum for null type representation.
@@ -86,18 +89,18 @@ class ColumnNullType(enum.IntEnum):
 class ColumnBuffers(TypedDict):
     # first element is a buffer containing the column data;
     # second element is the data buffer's associated dtype
-    data: Tuple["Buffer", Any]
+    data: Tuple["Buffer", Dtype]
 
     # first element is a buffer containing mask values indicating missing data;
     # second element is the mask value buffer's associated dtype.
     # None if the null representation is not a bit or byte mask
-    validity: Optional[Tuple["Buffer", Any]]
+    validity: Optional[Tuple["Buffer", Dtype]]
 
     # first element is a buffer containing the offset values for
     # variable-size binary data (e.g., variable-length strings);
     # second element is the offsets buffer's associated dtype.
     # None if the data buffer does not have an associated offsets buffer
-    offsets: Optional[Tuple["Buffer", Any]]
+    offsets: Optional[Tuple["Buffer", Dtype]]
 
 
 class CategoricalDescription(TypedDict):
@@ -237,7 +240,7 @@ def offset(self) -> int:
 
     @property
     @abstractmethod
-    def dtype(self) -> Tuple[DtypeKind, int, str, str]:
+    def dtype(self) -> Dtype:
         """
         Dtype description as a tuple ``(kind, bit-width, format string, endianness)``.