Tsrc

* Ad more guides in the documentation
* Display absolute paths when performing file system operations
* `tsrc sync` now uses parallel jobs by default. Use `-j1` to force
sequential processing. Patch by gdubicki

Highlights

* All of `tsrc` commands can now be run in parallel. Try for instance `tsrc sync -j auto`.

* `tsrc foreach` now sets a bunch of environment variables. This allows
developers to add new behaviors to tsrc without having to change its source code. See
the [relevant guide](https://dmerejkowsky.github.io/tsrc/guide/foreach) for more information.

* Augment documentation with more use cases and examples (still a work in progress)

Breaking changes

* Remove `tsrc version` - Use `tsrc --version` instead.

* The 'parallel' feature caused the output of some commands like
`foreach` or `log` to change slightly. Hopefully
`tsrc` output is now more consistent.

Bug fixes

* Fix crash when running `tsrc` without any arguments

* Fix crash when trying to clone repositories in some rare corner cases
(like the destination existing but not being a directory)

Internal changes

* Make all `tsrc` imports consistent
* Fix error when calling `repr` on `tsrc` Errors.

Bug fixes and small improvements

* Fix 268: `tsrc apply-manifest` now performs file system operation
* Always display workspace path at the beginning of any action
* Skip "performing file system operations" message if there is no
work to be done

Internal changes

* Bump linters and formatters (`black`, `mypy`, `isort` ...)
* Use `copier` to simplify maintenance of tools configuration

Repo selection

* Added -r (regex) and -i (inverse regex) params for filtering repos. Patch by xzr

Add support for submodules

* `tsrc` calls `git clone` with `--recurse-submodules` when adding missing repositories
* `tsrc` calls `git submodule update --init --recursive` when updating repositories

Misc

* Remove codecov usage
* Rename default branch to `main`.

* Project has been moved from `TankerHQ` organization to `dmerejkowsky`. New urls are:

* [github.com/dmerejkowsky/tsrc](https://github.com/dmerejkowsky/tsrc) for the git repository
* [dmerejkowsky.github.io/tsrc](https://dmerejkowsky.github.io/tsrc) for the documentation

* Add CI jobs to check this project also works with Python 3.9
* Drop ``Path Pie`` dependency
* Minor internal fixes
* Add more URLs in the metadata (and pypi.org project page)

Add symlink support

`tsrc sync` and `tsrc init` can now create symlinks as specified in the manifest file:

yaml
repos:

- url: gitgitlab.local:proj1/app
dest: app
symlink:
- source: app/some_file
target: ../some_file


In this case, a symlink will be created from `<workspace>/app/some_file` to `<workspace>/some_file`.
(both `source` and `target` keys are relative to the repository's destination).

Changes related to groups

* log, status, and sync all learned about the `--group` option and the `--all-cloned` options
* foreach: remove the `--groups-from-config` options since this is now the default behavior

Misc

* Rework FAQ
* Run black in `lint.sh`
* Fix formatting of some messages
* Update code manifesto to suggest using docstrings in tests