Fix imageseries test warnings, update back compat (#1553)

Author: rly

src/pynwb/image.py

@@ -97,7 +97,7 @@ def __init__(self, **kwargs):
             warnings.warn(
                 "%s '%s': The value for 'format' has been changed to 'external'. "
                 "Setting a default value for 'format' is deprecated and will be changed "
-                "to raising a ValueError in the next release."
+                "to raising a ValueError in the next major release."
                 % (self.__class__.__name__, self.name),
                 DeprecationWarning,
             )

tests/back_compat/1.5.1_imageseries_no_data.nwb

@@ -52,6 +52,7 @@ def test_external_file_no_frame(self):
             name='test_iS',
             unit='unit',
             external_file=['external_file'],
+            format="external",
             timestamps=[1., 2., 3.]
         )
         self.assertListEqual(iS.starting_frame, [0])
@@ -78,6 +79,7 @@ def test_external_file_no_unit(self):
         iS = ImageSeries(
             name='test_iS',
             external_file=['external_file'],
+            format="external",
             timestamps=list()
         )
         self.assertEqual(iS.unit, ImageSeries.DEFAULT_UNIT)
@@ -242,7 +244,7 @@ def test_external_file_default_format(self):
         msg = (
             "ImageSeries 'test_iS': The value for 'format' has been changed to 'external'. "
             "Setting a default value for 'format' is deprecated and will be changed to "
-            "raising a ValueError in the next release."
+            "raising a ValueError in the next major release."
         )
         with self.assertWarnsWith(DeprecationWarning, msg):
             iS = ImageSeries(