Simplify warning test

skimage/filter/__init__.py

@@ -1,12 +1,14 @@
 from skimage._shared.utils import skimage_deprecation
-from warnings import warn_explicit
-__warningregistry__ = {}
-warn_explicit(skimage_deprecation('The `skimage.filter` module has been renamed to '
-                                  '`skimage.filters`.  This placeholder module '
-                                  'will be removed in v0.13.'), category=None,
-                                  filename=__file__, lineno=4,
-                                  registry=__warningregistry__)
-del warn_explicit
+from warnings import warn
+
+global _import_warned
+
+warn(skimage_deprecation('The `skimage.filter` module has been renamed '
+                         'to `skimage.filters`.  This placeholder module '
+                         'will be removed in v0.13.'))
+_import_warned = True
+
+del warn
 del skimage_deprecation
 
 from ..filters import *

skimage/filters/tests/test_filter_import.py

@@ -3,9 +3,8 @@ from warnings import catch_warnings, simplefilter
 
 def test_filter_import():
     with catch_warnings():
-        simplefilter('always')
+        simplefilter('ignore')
         from skimage import filter as F
 
     assert('sobel' in dir(F))
-    assert any(['has been renamed' in w
-                for (w, _, _) in F.__warningregistry__])
+    assert F._import_warned