Make methods of abstract base class async

The rationale is that the methods of child classes should match the
signatures of the methods of the abstract base class, including async.

src/zarr/abc/store.py

@@ -330,17 +330,17 @@ def supports_listing(self) -> bool:
         ...
 
     @abstractmethod
-    def list(self) -> AsyncGenerator[str, None]:
+    async def list(self) -> AsyncGenerator[str, None]:
         """Retrieve all keys in the store.
 
         Returns
         -------
         AsyncGenerator[str, None]
         """
-        ...
+        yield ""
 
     @abstractmethod
-    def list_prefix(self, prefix: str) -> AsyncGenerator[str, None]:
+    async def list_prefix(self, prefix: str) -> AsyncGenerator[str, None]:
         """
         Retrieve all keys in the store that begin with a given prefix. Keys are returned with the
         common leading prefix removed.
@@ -353,10 +353,10 @@ def list_prefix(self, prefix: str) -> AsyncGenerator[str, None]:
         -------
         AsyncGenerator[str, None]
         """
-        ...
+        yield ""
 
     @abstractmethod
-    def list_dir(self, prefix: str) -> AsyncGenerator[str, None]:
+    async def list_dir(self, prefix: str) -> AsyncGenerator[str, None]:
         """
         Retrieve all keys and prefixes with a given prefix and which do not contain the character
         “/” after the given prefix.
@@ -369,7 +369,7 @@ def list_dir(self, prefix: str) -> AsyncGenerator[str, None]:
         -------
         AsyncGenerator[str, None]
         """
-        ...
+        yield ""
 
     def close(self) -> None:
         """Close the store."""