utils: drop make_ui parameter clear_session - this utility function should never mess with sessions clear_session seems to have been based on the idea that utility functions that happened to create a session also should clean up. But instead, we now consistently take care of session removal at a high level.

# 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
.