tests: admin_users: make sure repo group is deleted test_delete_repo_group_err creates then deletes a repository group. However, if the delete fails the repository group remains. This later causes problems in the model tests. Introduce a pytest yield fixture to handle the creation _and_ deletion of the repository group (suggested by Søren Løvborg). The creation of the user needs to happen _before_ that of the user group, and we cannot share data between two pytest fixtures, so the user is created in the fixture as well.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.