Consider removing pre-installed Python packages

[humitos]

We are installing some scientific Python packages,

pip install --no-cache-dir --only-binary numpy,scipy numpy scipy
pip install --no-cache-dir pandas matplotlib

This was useful some time ago to reduce compiling time on builds. Although, this has changed and all of them have wheel packages and they are not needed to be compiled anymore.

By installing them in a clean venv locally, it took just 10s.

I propose to remove them completely from the docker image.

Besides, I'd say that people depending on these packages they will install the exact version they want and not an old one that we can provide from our docker image.

Other considerations,

• by removing them, python.use_system_site_packages is probably no needed anymore and we could remove it from our YAML config
• docs have to be updated:
  • https://docs.readthedocs.io/en/latest/guides/build-using-too-many-resources.html?highlight=numpy#use-system-site-packages-for-pre-installed-libs
  • https://docs.readthedocs.io/en/latest/builds.html?highlight=numpy#stable
  • https://docs.readthedocs.io/en/latest/config-file/v1.html?highlight=numpy#python-use-system-site-packages

Ref #1

[humitos]

3.9.0rc1 won't come with pre-installed requirements, see #137

[humitos]

New Docker (#166) images won't come with these pre-installed packages.