fix: Make warnings module optional in __del__ to handle python 3.8

Author: bhatt4982

spannerlib/wrappers/spannerlib-python/spannerlib-python/google/cloud/spannerlib/internal/message.py

@@ -154,7 +154,7 @@ def release(self) -> None:
             logger.error("Unexpected error during release: %s", e)
             # We do not re-raise here to ensure __exit__ completes cleanly
 
-    def __del__(self) -> None:
+    def __del__(self, _warnings=warnings) -> None:
         """Finalizer: The Safety Net.
 
         Checks if the resource was leaked. If so, issues a ResourceWarning
@@ -164,15 +164,15 @@ def __del__(self) -> None:
         if getattr(self, "pinner_id", 0) != 0 and not getattr(
             self, "_is_released", False
         ):
-            try:
-                warnings.warn(
-                    "Unclosed SpannerLib Message"
-                    f"(pinner_id={self.pinner_id})",
-                    ResourceWarning,
-                    source=self,
-                )
-            except Exception:
-                pass
+            if _warnings is not None:
+                try:
+                    _warnings.warn(
+                        "Unclosed SpannerLib Message"
+                        f"(pinner_id={self.pinner_id})",
+                        ResourceWarning,
+                    )
+                except Exception:
+                    pass
 
             try:
                 self.release()