[Bug]: Upgrade issue - `jupyter book` command doesn't enter the `Found a legacy Jupyter Book` process

[da5nsy]

Closing this PR on the-turing-way repo and moving it to here since this seems to be a JB issue rather than a TTW issue

Quick summary (see the PR linked above for further details): I tried to upgrade the book on Windows and for some reason on my machine it isn't recognising that it is an existing book.

Linking to: https://github.com/jupyter-book/mystmd/issues/1336

Version

2.0.0a0

What Operating System are you using?

Windows

Relevant log output

https://github.com/the-turing-way/the-turing-way/pull/3958#issuecomment-2489542621