remove databases, use SQLAlechemy 2.0 instead

Among the many changes we switch to using SQLAlchemy's connection pool,
which means we are no longer required to guard against multiple threads
working on the database.
All db funcs now receive a connection to use as their first argument,
this allows the caller to control transaction & rollback behavior.

tests/test_models.py

@@ -0,0 +1,11 @@
+import pytest
+
+from unwind import models
+
+
+@pytest.mark.parametrize("mapper", models.mapper_registry.mappers)
+def test_fields(mapper):
+    """Test that models.fields() matches exactly all table columns."""
+    dcfields = {f.name for f in models.fields(mapper.class_)}
+    mfields = {c.name for c in mapper.columns}
+    assert dcfields == mfields