Incompatibility between WTForms<=2.1 and SQLAlchemy>=1.2

[magical]

Older versions of WTForms are not compatible with SQLAlchemy 1.2 (pallets-eco/wtforms-sqlalchemy#9). We should probably declare a dependency on WTForms>=2.2.

But first we should make sure everything works correctly under WTForms 2.3 (the latest release).