misc cleanup set([])

src/calibre/gui2/dialogs/device_category_editor.py

@@ -60,7 +60,7 @@ def __init__(self, window, tag_to_match, data, key):
         self.setWindowIcon(icon)
 
         self.to_rename = {}
-        self.to_delete = set([])
+        self.to_delete = set()
         self.original_names = {}
         self.all_tags = {}
 

src/calibre/gui2/dialogs/scheduler.py

@@ -587,7 +587,7 @@ def __init__(self, parent, db):
         self.recipe_model = RecipeModel()
         self.db = db
         self.lock = QMutex(QMutex.Recursive)
-        self.download_queue = set([])
+        self.download_queue = set()
 
         self.news_menu = QMenu()
         self.news_icon = QIcon(I('news.png'))

src/calibre/gui2/dialogs/select_formats.py

@@ -81,7 +81,7 @@ def __init__(self, fmt_count, msg, single=False, parent=None, exclude=False):
         self.fview.setSpacing(2)
 
         self.resize(350, 500)
-        self.selected_formats = set([])
+        self.selected_formats = set()
 
     def accept(self, *args):
         for idx in self.fview.selectedIndexes():