Use setuptools-scm to extract the version number

Add nested test

Do not set rootpath when using prefix_with_layout

✨ add a way to use expanded projects’ layout as the base layout for subelements.

Update tox configuration and add all supported mercurial versions in it

concurrency: add a global 'jobs' config item

ensureconf: implement concurrency

forge: more generic forge types registry

forge: fetch .tar.gz files instead of .zip

Initial implementation of 'cfensureconf --ci'

ensureconf: fix last regressions, most notably on tarball clone/update

commands: Refactor the cfensureconf algorithm to allow concurrency & future ci mode

Non conservative change:
The 'xxx is a partial override without its master entry' warning message has
been completely removed.

configuration: small reorganization of the clone and checkout code

The goal is to make reuse easier later on.

configuration: remove the destination directory if clone fails

Add a 'cfclear' command

Ignore a few test/build files

Configure and apply isort & black

doc: document the 'hggit' config item

add hggit as a global option

Add a config item '[confman] hggit' for changing the default value of the 'hggit' command flag.