Use explicit type check instead of duck typing

See #406 (comment)

CHANGELOG.rst

@@ -1,6 +1,18 @@
 Changelog
 ---------
 
+5.4.0 (unreleased)
+******************
+
+Changes:
+
+* Use explicit type check for `fields.DelimitedList` when deciding to
+  parse value with `getlist()` (`#406 (comment) <https://github.com/marshmallow-code/webargs/issues/406#issuecomment-514446228>`_ ).
+
+Support:
+
+* Add "Parsing Lists in Query Strings" section to docs (:issue:`406`).
+
 5.3.2 (2019-06-19)
 ******************
 

src/webargs/core.py

@@ -65,7 +65,11 @@ class Meta(object):
 
 def is_multiple(field):
     """Return whether or not `field` handles repeated/multi-value arguments."""
-    return isinstance(field, ma.fields.List) and not hasattr(field, "delimiter")
+    from webargs import fields  # prevent circular import
+
+    return isinstance(field, ma.fields.List) and not isinstance(
+        field, fields.DelimitedList
+    )
 
 
 def get_mimetype(content_type):