This is not a matter of Python 2 vs Python 3. It builds identically AFAIK. (Although RTD uses 2.7: do we know where to set that?)

The motivation for pinning sphinx to <2.0 for now was this RTD defect: readthedocs/sphinx_rtd_theme#739, so as to avoid this ugly warning:

...\jythonbook\venv\lib\site-packages\sphinx_rtd_theme\search.html:20: RemovedInSphinx30Warning: To modify script_files in the theme is deprecated. Please insert a <script> tag directly in your theme instead.
  {{ super() }}

It's fixed, but not released. pip install --upgrade sphinx_rtd_theme still gives me 0.4.3 which contains it. When the fix is released, we could undo the pin unconditionally, I believe.

Understood. Thanks. Closing...