refactor: remove cycle in processor stats (#13204)

We remove a static circular import on typing in the processor stats
module.

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Author: P403n1x87

ddtrace/internal/processor/stats.py

@@ -27,7 +27,7 @@
     from typing import Optional  # noqa:F401
     from typing import Union  # noqa:F401
 
-    from ddtrace.trace import Span  # noqa:F401
+    from ddtrace._trace.span import Span  # noqa:F401
 
 
 log = get_logger(__name__)