Add type hints for pageLength

[mwouts]

Closes #410

[github-actions]

Thank you for making this pull request.

Did you know? You can try it on Binder: .

Also, the version of ITables developed in this PR can be installed with pip:

pip install git+https://github.com/mwouts/itables.git@add_page_length

(this requires nodejs, see more at Developing ITables)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.77%. Comparing base (3b06b96) to head (e01f0cc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #413      +/-   ##
==========================================
+ Coverage   87.76%   87.77%   +0.01%     
==========================================
  Files          50       50              
  Lines        2002     2004       +2     
==========================================
+ Hits         1757     1759       +2     
  Misses        245      245              

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