WIP: use copier

[thomaspaulb]

• When starting a new Waft project, use copier instead of git clone
• When updating an existing Waft project, use copier instead of editing and re-running bootstrap
• When updating, don't override local README.md
• Check how to make copier pick as a default the answers that were chosen last time.
• When turning an existing Waft project into a copier project by using "copier copy" over the thing, guess default answers to questions based on the files that are already there, using context hooks.
• Test the use of migration scripts during copier update
• Try to use migration scripts to replace symlinks with actual copies, and if that is desirable.
• Test how we can do things that normally re-bootstrap did (eg update waftlib) --> spoke with Hussam about having bootstrap do "copier update --vcs-ref 1.x" and keeping an 1.x branch, so that Ansible can still apply minor patches by running bootstrap. Bigger patches will need a major version upgrade to eg 2.x and this bootstrap would not be able to do. This would however mean that what we now do in bootstrap, eg clone waftlib, will have to be moved to a secondary script eg _copier/bootstrap2 or something, which will get triggered by copier update.
• Test template versioning. How does it work? with git tags? Yes with tags, it looks for the latest tag. Although we can also rather use branches.
• Try to add for example a tool dependency in pyproject.toml, and see how that works on a project upgrade.