Simplify test skipping

Tests/test_file_webp_alpha.py

@@ -5,18 +5,13 @@ from PIL import Image
 try:
     from PIL import _webp
 except ImportError:
-    pass
-    # Skip in setUp()
+    _webp = None
 
 
+@unittest.skipIf(_webp is None, "WebP support not installed")
 class TestFileWebpAlpha(PillowTestCase):
 
     def setUp(self):
-        try:
-            from PIL import _webp
-        except ImportError:
-            self.skipTest('WebP support not installed')
-
         if _webp.WebPDecoderBuggyAlpha(self):
             self.skipTest("Buggy early version of WebP installed, "
                           "not testing transparency")

Tests/test_imagetk.py

@@ -18,11 +18,10 @@ except (OSError, ImportError) as v:
 TK_MODES = ('1', 'L', 'P', 'RGB', 'RGBA')
 
 
+@unittest.skipIf(not HAS_TK, "Tk not installed")
 class TestImageTk(PillowTestCase):
 
     def setUp(self):
-        if not HAS_TK:
-            self.skipTest("Tk not installed")
         try:
             # setup tk
             tk.Frame()

Tests/test_pyroma.py

@@ -8,9 +8,9 @@ except ImportError:
     pyroma = None
 
 
+@unittest.skipIf(pyroma is None, "Pyroma is not installed")
 class TestPyroma(PillowTestCase):
 
-    @unittest.skipUnless(pyroma, "Pyroma is not installed")
     def test_pyroma(self):
         # Arrange
         data = pyroma.projectdata.get_data(".")