Update Read the Docs configuration (automatic)

The following migrators were applied:

- Migrate to `build.tools` configuration.

This uses the new base Docker image based on Ubuntu 20.04 introduced in October 2021
and picks an appropriate Python version for your project
(read [our blog post](https://blog.readthedocs.com/new-build-specification/)
for details).
Notice that now you can specify the Node.js, Rust, and Go versions as well.

*Note:* Some system dependencies are not preinstalled anymore,
so this might require manually adding them to `build.apt_packages`
(see [our
documentation](https://docs.readthedocs.io/en/stable/config-file/v2.html#build-apt-packages>)).

- Migrate to Mamba as a drop-in replacement for Conda.

Your project requested using Mamba instead of Conda for performance reasons.
Now this is included in your configuration
and you can change it without our intervention.