GH-12: [Python] Fix invalid-return-type error (#25)

* Add py.typed file to signify that the library is typed

See the relevant PEP https://peps.python.org/pep-0561

* Prepare `pyarrow-stubs` for history merging

MINOR: [Python] Prepare `pyarrow-stubs` for history merging

Co-authored-by: ZhengYu, Xu <zen-xu@outlook.com>

* Add `ty` configuration and suppress error codes

* One line per rule

* Add licence header from original repo for all `.pyi` files

* Revert "Add licence header from original repo for all `.pyi` files"

This reverts commit 1631f39.

* Prepare for licence merging

* Exclude `stubs` from `rat` test

* Add Apache licence clause to `py.typed`

* Reduce list

* Add `ty` as a step in the action

* Run in the correct directory

* Remove `check` from `pip`

* Fix `unresolved-reference` error

* Revert "Fix `unresolved-reference` error"

This reverts commit 7ee3d2f.

* Fix `invalid-return-type` error

* Fix linting

---------

Co-authored-by: Rok Mihevc <rok@mihevc.org>

Author: paddyroddy

python/pyarrow/interchange/column.py

@@ -20,7 +20,6 @@
 import enum
 from typing import (
     Any,
-    Dict,
     Iterable,
     Optional,
     Tuple,
@@ -379,7 +378,7 @@ def describe_null(self) -> Tuple[ColumnNullType, Any]:
             return ColumnNullType.USE_BITMASK, 0
 
     @property
-    def null_count(self) -> int:
+    def null_count(self) -> int | None:
         """
         Number of null elements, if known.
 
@@ -390,7 +389,7 @@ def null_count(self) -> int:
         return n
 
     @property
-    def metadata(self) -> Dict[str, Any]:
+    def metadata(self) -> None:
         """
         The metadata for the column. See `DataFrame.metadata` for more details.
         """
@@ -466,7 +465,7 @@ def get_buffers(self) -> ColumnBuffers:
 
     def _get_data_buffer(
         self,
-    ) -> Tuple[_PyArrowBuffer, Any]:  # Any is for self.dtype tuple
+    ) -> Tuple[_PyArrowBuffer, Any] | None:  # Any is for self.dtype tuple
         """
         Return the buffer containing the data and the buffer's
         associated dtype.
@@ -505,7 +504,7 @@ def _get_validity_buffer(self) -> Tuple[_PyArrowBuffer, Any]:
                 "There are no missing values so "
                 "does not have a separate mask")
 
-    def _get_offsets_buffer(self) -> Tuple[_PyArrowBuffer, Any]:
+    def _get_offsets_buffer(self) -> Tuple[_PyArrowBuffer, Any] | None:
         """
         Return the buffer containing the offset values for variable-size binary
         data (e.g., variable-length strings) and the buffer's associated dtype.