fixup! Update RefTrace's garbage collection

Author: ajaust

python/segyio/trace.py

@@ -412,13 +412,6 @@ class RefTrace(Trace):
     :attr:`Trace.ref`. This feature requires a context manager, to guarantee
     modifications are written back to disk.
     """
-
-    # If there are no external references to the data (so only internal
-    # references remain), the reference count is
-    # - 2 (Python >= 3.14)
-    # - 3 (Python < 3.14)
-    _GARBAGE_THRESHOLD = 3 if sys.version_info < (3, 14) else 2
-
     def __init__(self, *args):
         super(RefTrace, self).__init__(*args)
         self.refs = {}
@@ -442,6 +435,11 @@ def flush(self):
         be useful in certain contexts to provide stronger guarantees.
         """
         garbage = []
+        # If there are no external references to the data (so only internal
+        # references remain), the reference count is
+        # - 2 (Python >= 3.14)
+        # - 3 (Python < 3.14)
+        garbage_threshold = 3 if sys.version_info < (3, 14) else 2
 
         for i, (x, signature) in self.refs.items():
             if sys.getrefcount(x) == self._GARBAGE_THRESHOLD: