db: upgrade to SQLAlchemy 1.1, fixing invalid PostgreSQL SQL in User.is_default_user eea19c23b741 triggered a bug in SQLAlchemy 1.0.17, causing invalid SQL to be emitted for PostgreSQL: For the (Python) query '(x == y) == z', SQLAlchemy generates the SQL 'x = y = z'. SQLite and MySQL accepts this, but PostgreSQL does not. This bug has been fixed in 1.1. Testing and review shows no problems with 1.1

# 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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