borrow is_nonstr_iter from pyramid and use it instead of simple isinstance check

colander/__init__.py

@@ -11,6 +11,7 @@ from .compat import (
     text_type,
     string_types,
     xrange,
+    is_nonstr_iter,
     )
 
 from . import iso8601
@@ -75,9 +76,9 @@ class Invalid(Exception):
         ``msg`` attribute is iterable, it is returned.  If it is not
         iterable, a single-element list containing the ``msg`` value
         is returned."""
-        if isinstance(self.msg, string_types):
-            return [self.msg]
+        if is_nonstr_iter(self.msg):
             return self.msg
+        return [self.msg]
 
     def add(self, exc, pos=None):
         """ Add a child exception; ``exc`` must be an instance of

colander/compat.py

@@ -16,6 +16,15 @@ def text_(s, encoding='latin-1', errors='strict'):
         return s.decode(encoding, errors)
     return s # pragma: no cover
 
+if PY3: # pragma: no cover
+    def is_nonstr_iter(v):
+        if isinstance(v, str):
+            return False
+        return hasattr(v, '__iter__')
+else:
+    def is_nonstr_iter(v):
+        return hasattr(v, '__iter__')
+
 try:
     xrange = xrange
 except NameError: # pragma: no cover