fix proxy issue

Author: zm711

neo/core/spiketrain.py

@@ -24,7 +24,8 @@
 import quantities as pq
 from neo.core.baseneo import BaseNeo, MergeError, merge_annotations
 from neo.core.dataobject import DataObject, ArrayDict
-from neo.io.proxyobjects import SpikeTrainProxy
+# need this to avoid circular import issue
+import neo.io
 
 
 def check_has_dimensions_time(*values) -> None:
@@ -817,7 +818,7 @@ def merge(self, *others):
         compatible, an Exception is raised.
         """
         for other in others:
-            if isinstance(other, SpikeTrainProxy):
+            if isinstance(other, neo.io.SpikeTrainProxy):
                 raise MergeError(
                     "Cannot merge, SpikeTrainProxy objects cannot be merged"
                     "into regular SpikeTrain objects, please load them first."