Fix missing _() in some cases.

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ERROR: test_display_tag_name (test.test_utils.TagsTest)
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Traceback (most recent call last):
File "/home/travis/build/musicbrainz/picard/test/test_utils.py", line 89, in test_display_tag_name
self.assertEqual(dtn('tag'), 'tag')
File "/home/travis/build/musicbrainz/picard/picard/util/tags.py", line 106, in display_tag_name
return _(name)
NameError: global name '_' is not defined