misc cleanup

src/calibre/ebooks/oeb/reader.py

@@ -233,7 +233,7 @@ class OEBReader(object):
                         if not scheme and href not in known:
                             new.add(href)
             unchecked.clear()
-            warned = set([])
+            warned = set()
             for href in new:
                 known.add(href)
                 is_invalid = False

src/calibre/ebooks/oeb/transforms/filenames.py

@@ -97,7 +97,7 @@ class UniqueFilenames(object):  # {{{
         self.opts = opts
         self.oeb = oeb
 
-        self.seen_filenames = set([])
+        self.seen_filenames = set()
         self.rename_map = {}
 
         for item in list(oeb.manifest.items):