Close sample input file after reading it

Otherwise tests spit out:

    ResourceWarning: unclosed file <_io.TextIOWrapper name='/Users/pypt/Dropbox/etc-MediaCloud/python-readability/tests/samples/si-game.sample.html' mode='r' encoding='UTF-8'>
    return open(os.path.join(SAMPLES, filename)).read()

tests/test_article_only.py

@@ -9,7 +9,9 @@ SAMPLES = os.path.join(os.path.dirname(__file__), 'samples')
 
 def load_sample(filename):
     """Helper to get the content out of the sample files"""
-    return open(os.path.join(SAMPLES, filename)).read()
+    with open(os.path.join(SAMPLES, filename)) as f:
+        html = f.read()
+    return html
 
 
 class TestArticleOnly(unittest.TestCase):