bump version function doesn't work

### What went wrong?

Since `TokenListVersion` is a frozen model, `TokenList.bump_version` fails

### How can it be fixed?

Make it not frozen, no reason for it to be

Start at the TokenListVersion model and the TokenList.bump_version function named in the issue. Confirm how the frozen model causes bump_version to fail, then verify that bump_version succeeds after the model is made mutable and that existing behavior remains intact.

Written by the indexing model from the issue text.