install all Python dependencies in read-the-docs build environment

Read the Docs used to pre-install common scientific Python packages like scipy, numpy, pandas, matplotlib and others at system level to speed up the build process. However, with all the work done in the Python ecosystem and the introduction of "wheels", these packages are a lot easier to install via pip install and these pre-installed packages are not required anymore. If you have Apt package dependencies, they can be installed with [build.apt_packages](https://docs.readthedocs.io/en/stable/config-file/v2.html#build-apt-packages).

With the introduction of our new "Ubuntu 20.04" and "Ubuntu 22.04" Docker images, we stopped pre-installing these extra Python packages and we encouraged users to install and pin all their dependencies using a requirements.txt file. We have already stopped supporting "use system packages" on these newer images.

We are removing the "use system packages" feature on August 29th. Make sure you are installing all the required dependecies to build your project's documentation using a requirements.txt file and specifying it in your .readthedocs.yaml.

Here you have an example of the section required on the .readthedocs.yaml configuration file:

python:
install:
- requirements: docs/requirements.txt
The content of docs/requirements.txt would be similar to:

scipy==1.11.1
numpy==1.25.2
pandas==2.0.3
matplotlib==3.7.2
We are sending you this email because you are a maintainer of the following projects that are affected by this removal. Either using "Use system package" checkbox in the Admin UI, or the config key python.sytem_packages or python.use_system_site_packages in your .readthedocs.yaml file:

[simpleitk-notebooks](https://readthedocs.org/projects/simpleitk-notebooks/)
Read more about this in our [Reproducible builds](https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html) guide for more details.

Start by checking the repository's .readthedocs.yaml and whether docs/requirements.txt exists; compare the configured Python installation with the dependencies used by the notebooks and documentation. Run the Read the Docs build or its documented local equivalent, and consider the work done when all required packages are explicitly pinned and the documentation build completes without system packages.

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