fix: upgrade sql requirements to support the online mode

[ncrocfer]

This PR switches to online mode in order to fix https://github.com/ovh/celery-director/issues/164

After merging it we'll be able to review & merge #170 and #162 (these PRs introduce new migrations).

POC:

$ cat director/migrations/versions/ea91724fbbb9_add_foo.py
"""add foo

Revision ID: ea91724fbbb9
Revises: 46e4acde004e
Create Date: 2022-11-22 16:38:13.759083

"""
from alembic import op
import sqlalchemy as sa

# revision identifiers, used by Alembic.
revision = 'ea91724fbbb9'
down_revision = '46e4acde004e'
branch_labels = None
depends_on = None

def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.add_column('workflows', sa.Column('foo', sa.String(length=255), nullable=False))
    # ### end Alembic commands ###

def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('workflows', 'foo')
    # ### end Alembic commands ###

$ director db upgrade
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade  -> 30d6f6636351, Initial migration
INFO  [alembic.runtime.migration] Running upgrade 30d6f6636351 -> 05cf96d6fcae, Add task result
INFO  [alembic.runtime.migration] Running upgrade 05cf96d6fcae -> 3f8466b16023, Add users table
INFO  [alembic.runtime.migration] Running upgrade 3f8466b16023 -> 063ff371f2da, Add index on workflow_id in task table
INFO  [alembic.runtime.migration] Running upgrade 063ff371f2da -> 2ac615d6850b, Force varchar 255
INFO  [alembic.runtime.migration] Running upgrade 2ac615d6850b -> 46e4acde004e, Add cascade in task model for workflow deletion
INFO  [alembic.runtime.migration] Running upgrade 46e4acde004e -> ea91724fbbb9, add foo

$ director db upgrade 46e4acde004e:ea91724fbbb9 --sql
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Generating static SQL
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 46e4acde004e -> ea91724fbbb9, add foo
-- Running upgrade 46e4acde004e -> ea91724fbbb9

ALTER TABLE workflows ADD COLUMN foo VARCHAR(255) NOT NULL;

UPDATE alembic_version SET version_num='ea91724fbbb9' WHERE alembic_version.version_num = '46e4acde004e';

Tested with SQLite & PostgreSQL.