This PR brings changes to the CI:

• The main tests install now the dependencies using pip. Conda tests are run on a scheduled base, on releases and on part of downstream tests.
• When the docs are built with conda, another job also builds them - without publishing - with pip.

Overall, I was impressed by how things improved in the pip world, most packages can be installed without any issue on the 3 platforms. Some remaining issues:

• pygraphviz has no wheel and requires some special setup to build from the sdist. Since it's just required to build the user_guide/NetworkX.ipynb notebook, I've put it in its own graphviz extra, that is only pulled when the docs/tests jobs run with conda.
• cartopy cannot be installed on Python 3.8 on Ubuntu and Windows. I've created a geo extra to deal with that similarly to graphviz. Starting from 3.9 it looks like it will no longer be required!

When these dependencies are not installed, the notebooks that need them are skipped.