Make methods of abstract base class async

The goal is that the async methods of child classes match the signatures
of the methods of the abstract base class.

src/zarr/abc/store.py

@@ -329,7 +329,7 @@ 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
@@ -339,7 +339,7 @@ def list(self) -> AsyncGenerator[str, None]:
         ...
 
     @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.
@@ -355,7 +355,7 @@ def list_prefix(self, prefix: str) -> AsyncGenerator[str, None]:
         ...
 
     @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.