Add get_stats to MemoryBackend and update pytest config

Added get_stats() method for consistency with RedisBackend. Also added
httpx and pytest-asyncio as dev dependencies.

fastapi_traffic/backends/memory.py

@@ -134,6 +134,19 @@ class MemoryBackend(Backend):
             self._cleanup_task = None
         await self.clear()
 
+    async def ping(self) -> bool:
+        """Check if the backend is available. Always returns True for memory backend."""
+        return True
+
+    async def get_stats(self) -> dict[str, Any]:
+        """Get statistics about the rate limit storage."""
+        async with self._lock:
+            return {
+                "total_keys": len(self._data),
+                "max_size": self._max_size,
+                "backend": "memory",
+            }
+
     def __len__(self) -> int:
         """Return the number of stored entries."""
         return len(self._data)