db: drop double specification of unique constraint (Issue #374) Consistently use "unique=True" for single column constraints. Only use UniqueConstraint for composite constraints. MySQL did not like this double specification, while PostgreSQL and SQLite did not care.

# 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
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