Close file after finished reading in ImageFont._load_pilfont()

Fixes some "ResourceWarning: unclosed file ..." when running tests with
warnings enabled.

PIL/ImageFont.py

@@ -62,8 +62,7 @@ class ImageFont(object):
 
     def _load_pilfont(self, filename):
 
-        fp = open(filename, "rb")
-
+        with open(filename, "rb") as fp:
             for ext in (".png", ".gif", ".pbm"):
                 try:
                     fullname = os.path.splitext(filename)[0] + ext