Add missing alter migration for unsubscribe

[panosangelopoulos]

@pyepye thanks for providing this wonderful library really great work.

I just found a missing migration and provide it via PR. Feel free to take a look and merge it if you find it useful.

[sultaniman]

Would be nice to have it.

[dosdos]

It would be great to merge this one @pyepye ! The makemigrations command is returning Uncommitted migrations found Thanks 🙏