Update tools for copier based swh-py-template

• Add copier as a dependency
• Adapt bin/init-py-repo to:
  • use copier to initialize the new swh project
  • create and initialize the git repo
  • optionnally create the corresponding gitlab project
• Remove the bin/cherry-py-repo-from-template since this is now the job of copier itself.
• Adapt the main Makefile to add '-I swh-docs' when needed to build the doc from sources.

Depends on swh-py-template!28 (merged)