Update tinytag.py

[armsby]

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Added extra tags for audiobooks Series this tag contains the series name Series_number this tag contains the number in the series

[devsnd]

Hey @armsby,

I cannot merge your PR because it has failing tests. Please make sure that the tests run through, and I'll give it another look. Also, please add the series number and series to the extra field, by assigning them to e.g. extra.series_number instead of series_number. This removes the need to add any new attributes to the base TinyTag instance too.

[devsnd]

I just wanted to port your patch to the newest version of tinytag, but apparently this is the "movement", like in classical music, and not the series/series number.

I also lack a test file to try this out, but once I have a test file, I can port your changes and add them to the extra fields.