removes some workarounds needed for sqlalchemy 1.1 + 2.0 support
1.4 backports most 2.0 behavior, keeping it off-by-default for an easier opt-in transition
opt-in with `session.future = True` flag
- avoid backref warnings by adding objects to session explicitly before creating any relationships
- remove unnecessary `[]` around scalar query
- use `text()` wrapper on connection.execute
- engine.execute is removed
- update import of declarative_base
- ensure RemovedIn20Warning is available for warnings filters on sqlalchemy < 1.4 (needs editable install to avoid pytest path mismatch)
- explicitly relay password in engine.url to alembic
removes need for our own implementation of the same behavior
but keep it around while we still support Python 3.6,
since the version (0.17) introducing asyncio_mode drops support for Python 3.6
