Rename Forecast.training_horizon into .train_horizon

- we use that shorter name in `urban_meal_delivery.forecasts.*`
  and want to be consistent in the ORM layer as well

migrations/versions/rev_20210202_12_8bfb928a31f8_rename_training_horizon_into_train_horizon.py

@@ -0,0 +1,48 @@
+"""Rename `Forecast.training_horizon` into `.train_horizon`.
+
+Revision: #8bfb928a31f8 at 2021-02-02 12:55:09
+Revises: #c2af85bada01
+"""
+
+import os
+
+from alembic import op
+
+from urban_meal_delivery import configuration
+
+
+revision = '8bfb928a31f8'
+down_revision = 'c2af85bada01'
+branch_labels = None
+depends_on = None
+
+
+config = configuration.make_config('testing' if os.getenv('TESTING') else 'production')
+
+
+def upgrade():
+    """Upgrade to revision 8bfb928a31f8."""
+    op.execute(
+        f"""
+        ALTER TABLE
+            {config.CLEAN_SCHEMA}.forecasts
+        RENAME COLUMN
+            training_horizon
+            TO
+            train_horizon;
+        """,
+    )  # noqa:WPS355
+
+
+def downgrade():
+    """Downgrade to revision c2af85bada01."""
+    op.execute(
+        f"""
+        ALTER TABLE
+            {config.CLEAN_SCHEMA}.forecasts
+        RENAME COLUMN
+            train_horizon
+            TO
+            training_horizon;
+        """,
+    )  # noqa:WPS355