fix: Import warnings in tracing

A missing `import warnings` in `tracing` is causing our linter action to fail

Author: szokeasaurusrex

sentry_sdk/tracing.py

@@ -1,6 +1,7 @@
 from datetime import datetime
 from enum import Enum
 import json
+import warnings
 
 from opentelemetry import trace as otel_trace, context
 from opentelemetry.trace import (