jupyter-book: Incorrect rendering of part titles in toc

Describe the bug

The part titles don’t seem to have emphasis applied to them.

To Reproduce

Steps to reproduce the behavior:

1. jupyter-book create test_book && cd test_book

2. _toc.yml

   - file: intro
   - part: first part
     chapters:
       - file: content
   

3. jupyter-book build .

Navigate to the documentation, observe

Expected behavior

The part title should be boldface, like at https://jupyterbook.org/

Environment

• Python Version: 3.8.3
• Package versions or output of jupyter-book --version:

Jupyter Book: 0.7.5  # also current master
MyST-NB: 0.10.0
Sphinx Book Theme: 0.0.36  # also current master
MyST-Parser: 0.12.7
Jupyter-Cache: 0.4.1

• Operating System: Ubuntu