[Bug]: Deprecated use of click.BaseCommand in cli/pluggable.py

[effigies]

What happened, and what did you expect to happen?

Running jupyter-book with warnings set to error raises a DeprecationWarning from click (see below).

The relevant code is present in main:

https://github.com/jupyter-book/jupyter-book/blob/5f18bfe70dd687f9f59b7dcc2d45cf2f50a01793/jupyter_book/cli/pluggable.py#L42-L52

What version of Jupyter Book are you running?

1.0.4.post1

What Operating System are you using?

Linux

Relevant log output

$ PYTHONWARNINGS="error" uv run jb build -W specification
Traceback (most recent call last):
  File "/[...]/.venv/bin/jb", line 4, in <module>
    from jupyter_book.cli.main import main
  File "/[...]/.venv/lib/python3.13/site-packages/jupyter_book/cli/main.py", line 19, in <module>
    from .pluggable import PluggableGroup
  File "/[...]/.venv/lib/python3.13/site-packages/jupyter_book/cli/pluggable.py", line 23, in <module>
    class PluggableGroup(click.Group):
    ...<28 lines>...
            return command
  File "/[...]/.venv/lib/python3.13/site-packages/jupyter_book/cli/pluggable.py", line 42, in PluggableGroup
    def get_command(self, ctx: click.Context, name: str) -> click.BaseCommand:
                                                            ^^^^^^^^^^^^^^^^^
  File "/[...]/.venv/lib/python3.13/site-packages/click/__init__.py", line 80, in __getattr__
    warnings.warn(
    ~~~~~~~~~~~~~^
        "'BaseCommand' is deprecated and will be removed in Click 9.0. Use"
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<2 lines>...
        stacklevel=2,
        ^^^^^^^^^^^^^
    )
    ^
DeprecationWarning: 'BaseCommand' is deprecated and will be removed in Click 9.0. Use 'Command' instead.