Pin jupyter-book<2 for docs build

[jeromekelleher]

Not sure how this slipped through - cropped up in tskit-dev/tskit-site#321

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.54%. Comparing base (6407cc1) to head (1271132).
⚠️ Report is 56 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
- Coverage   97.32%   95.54%   -1.78%     
==========================================
  Files           6        7       +1     
  Lines         337      382      +45     
  Branches       56       64       +8     
==========================================
+ Hits          328      365      +37     
- Misses          7        9       +2     
- Partials        2        8       +6     

Flag	Coverage Δ
python-tests	95.54% <ø> (?)
tszip	?

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