simplify building list of input/output formats

QComboBox.addItems() accepts any iterable, and there's no reason to use
map plus a list comprehension to operate on the same data.

- input_formats will currently be lowercase, output_formats is uppercase.
  Without documenting this inconsistent internal API, don't rely on it.

- While all builtin output plugins should have unicode_literals compatible
  file_type attributes, at least this assumption might break for custom
  plugins, so continue mapping to unicode_type.

Well, at least we can reduce the number of allocated lists.

src/calibre/gui2/convert/bulk.py

@@ -125,8 +125,7 @@ class BulkConfig(Config):
             preferred_output_format and preferred_output_format \
             in output_formats else sort_formats_by_preference(output_formats,
                     [prefs['output_format']])[0]
-        self.output_formats.addItems(list(map(unicode_type, [x.upper() for x in
-            output_formats])))
+        self.output_formats.addItems((unicode_type(x.upper()) for x in output_formats))
         self.output_formats.setCurrentIndex(output_formats.index(preferred_output_format))
 
     def accept(self):

src/calibre/gui2/convert/single.py

@@ -284,10 +284,8 @@ class Config(QDialog):
             preferred_output_format in output_formats else \
             sort_formats_by_preference(output_formats,
                     [prefs['output_format']])[0]
-        self.input_formats.addItems(list(map(unicode_type, [x.upper() for x in
-            input_formats])))
-        self.output_formats.addItems(list(map(unicode_type, [x.upper() for x in
-            output_formats])))
+        self.input_formats.addItems((unicode_type(x.upper()) for x in input_formats))
+        self.output_formats.addItems((unicode_type(x.upper()) for x in output_formats))
         self.input_formats.setCurrentIndex(input_formats.index(input_format))
         self.output_formats.setCurrentIndex(output_formats.index(preferred_output_format))