MP3 file extension should be case insensitive

[kk-hiraskar]

if file extension is anything apart from lowercase, like Mp3 or MP3 ...
then it's throwing error

mp3_tagger.exceptions.MP3OpenFileError: File must be MP3 format

https://github.com/artcom-net/mp3-tagger/blob/453ef8c22f688f8a28e95cd7298ea6ead60a0a4b/mp3_tagger/mp3.py#L87

[mibe]

Just FYI, this is a duplicate of #5.