misc: update readthedocs config (#4009)

Now required: https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

docs: update Sphinx version on RTD (#3882)

python3.9 -mvirtualenv sphinx
. sphinx/bin/activate
pip install Sphinx
pip freeze > requirements.txt

docs: update read-the-docs version of sphinx

This is specified both as best-practice to specify the defaults
explicitly, and as required for updating sphinx from v1.8.5

Refs: ht

docs: update read-the-docs version of sphinx

This is specified both as best-practice to specify the defaults
explicitly, and as required for updating sphinx from v1.8.5

Refs: https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
PR-URL: https://github.com/libuv/libuv/pull/3154
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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