Tsrc

* Additional bug fix for 165 - the fix in 0.9.1 was incomplete
* Improve error message when trying to use non-supported GitLab features (like using `tsrc push --reviewer`
on GitLab Community Edition)

* Improve error message when `tsrc foreach` fails to start the process. Suggested by dlewis-ald in 163
* Fix crash when finding reviewers for a GitLab project not in a group. Reported by irizzant in 165

* Add support for GitHub Enterprise: See the [relevant documentation](https://tankerhq.github.io/tsrc/guide/review/#handling_github_github_enterprise_pull_requests) for details - patch by sdavids13.
* Improve error message when using creating a merge request in a GitLab repository when the token cannot be found in the `tsrc` configuration file. Fix 158
* Fix crash when running `tsrc status` on a workspace with missing repositories (160) - reported by blastrock

* Implement `tsrc sync --force`. Currently, all it does is running `git fetch --force` on all repositories. Use with caution. See 152 for details.

* Fix crash in `tsrc sync` when the `repo` configuration in the manifest contained neither an URL nor a remote. `tsrc` now aborts as soon as the misconfiguration of the manifest is detected. See 154. (reported by jongep86)

* Add a `--file` option to `tsrc init` so that manifest can be read from a custom path in the file system
* Remove support for Python 3.4
* Switch from `xdg` to `pyxdg`
* Format the code with [black](https://github.com/python/black)