fix: issue #889 by changing the annotated return type of waiting_for to Self. (#890)

Fix issue #889 by changing the annotated return type of `waiting_for` to
`Self`.

Author: eteran

core/testcontainers/core/container.py

@@ -169,7 +169,7 @@ def maybe_emulate_amd64(self) -> Self:
             return self.with_kwargs(platform="linux/amd64")
         return self
 
-    def waiting_for(self, strategy: WaitStrategy) -> "DockerContainer":
+    def waiting_for(self, strategy: WaitStrategy) -> Self:
         """Set a wait strategy to be used after container start."""
         self._wait_strategy = strategy
         return self