fix: Fix BaseDatasetClient.iter_items type hints (#680)

Using syntax recommended in mypy docs:
https://mypy.readthedocs.io/en/stable/more_types.html#asynchronous-iterators

Author: Pijukatel

src/crawlee/base_storage_client/_base_dataset_client.py

@@ -122,6 +122,11 @@ async def iterate_items(
             An asynchronous iterator of dictionary objects, each representing a dataset item after applying
             the specified filters and transformations.
         """
+        # This syntax is to make mypy properly work with abstract AsyncIterator.
+        # https://mypy.readthedocs.io/en/stable/more_types.html#asynchronous-iterators
+        raise NotImplementedError
+        if False:  # type: ignore[unreachable]
+            yield 0
 
     @abstractmethod
     async def get_items_as_bytes(

src/crawlee/memory_storage_client/_dataset_client.py

@@ -203,7 +203,7 @@ async def list_items(
             )
 
     @override
-    async def iterate_items(  # type: ignore
+    async def iterate_items(
         self,
         *,
         offset: int = 0,

src/crawlee/storages/_dataset.py

@@ -402,7 +402,7 @@ async def iterate_items(
         Yields:
             Each item from the dataset as a dictionary.
         """
-        async for item in self._resource_client.iterate_items(  # type: ignore
+        async for item in self._resource_client.iterate_items(
             offset=offset,
             limit=limit,
             clean=clean,