db: clean up SQLAlchemy session flushes Many calls to Session().flush() were completely superfluous and have been removed. (See also the note on "flush" in the contributor docs.) For the remaining calls, a comment has been added to explain why it's necessary.

# 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
.