fix python reference to QUEUE

Author: MilesCranmer

pytest/test_all.py

@@ -115,7 +115,9 @@ def test_julia_gc():
             end
         end
         GC.gc()
-        @test isempty(PythonCall.GC.QUEUE.items)
+        Base.@lock PythonCall.GC.QUEUE begin
+            @test isempty(PythonCall.GC.QUEUE[])
+        end
         """
     )