fix: add missing typing on TracerProvider.shutdown (open-telemetry#4365)

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -1277,7 +1277,7 @@ def add_span_processor(self, span_processor: SpanProcessor) -> None:
         # SynchronousMultiSpanProcessor and ConcurrentMultiSpanProcessor.
         self._active_span_processor.add_span_processor(span_processor)
 
-    def shutdown(self):
+    def shutdown(self) -> None:
         """Shut down the span processors added to the tracer provider."""
         self._active_span_processor.shutdown()
         if self._atexit_handler is not None: