tests: push with force in test_vcs_operations.py _add_files_and_push Some tests push a fixed branch name. If such tests are run twice, it will fail second time when the other branch head already exists. Work around that by using force and thus replace the old head.

# 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
.