db: drop constraint that started failing with MariaDB 10.2 / MySQL 5.7 (Issue #324) The constraint was to prevent simple recursive parent references, but it only checked direct parents. We thus have to rely on the high level application maintaining the invariant anyway.

# 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
.