Fix Read the Docs builds

[kurtmckee]

Read the Docs builds began permanently failing on February 26, 2024.

The differential diagnosis between success and failure seems to be that the last successful build used Poetry 1.7.1; the first failing build used Poetry 1.8.1. For reference, Poetry 1.8.1 was released on February 26, 2024.

Read the Docs later updated its "Install dependencies with Poetry" documentation to note that Poetry needs a $VIRTUAL_ENV environment variable set.

This change aligns the Read the Docs config with the documented fix, and simultaneously updates the build OS and Python version, which are both very old.