Switch to new CI/CD configuration

We need to adapt this package to the new CI/CD and package format using citools:

• Modify pyproject.toml,
  • Add information from setup.py,
• Empty setup.py,
  • Leave the call to setup() for compatibility,
• Modify conda/meta.yaml,
  • Remove test and documentation commands,
  • Import names from pyproject.toml,
• Modify .gitlab-ci.yml,
  • Use the nicer format instead of the URL,
• Add the project path to PATH in doc/conf.py,
• Remove version.txt and put the version in pyproject.toml.

You can look at bob.learn.em!68 (merged) for an example of a ported package.