While it is not an important case it might be annoying to users that if they used airflow 1.10 with sqlite, the migration to 2.0 will fail on dropping constraints in `known_event` table. 

It would be great to provide some more useful message then asking the user to remove the sqlite database. 

```
[2021-01-24 08:38:42,015] {db.py:678} INFO - Creating tables
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 03afc6b6f902 -> cf5dc11e79ad, drop_user_and_chart
Traceback (most recent call last):
  File "/Users/vijayantsoni/.virtualenvs/airflow/bin/airflow", line 11, in <module>
    sys.exit(main())
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/cli/cli_parser.py", line 48, in command
    return func(*args, **kwargs)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/cli/commands/db_command.py", line 31, in initdb
    db.initdb()
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/utils/db.py", line 549, in initdb
    upgradedb()
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/utils/db.py", line 688, in upgradedb
    command.upgrade(config, 'heads')
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/command.py", line 294, in upgrade
    script.run_env()
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/script/base.py", line 481, in run_env
    util.load_python_file(self.dir, "env.py")
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/util/pyfiles.py", line 97, in load_python_file
    module = load_module_py(module_id, path)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/util/compat.py", line 182, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/migrations/env.py", line 108, in <module>
    run_migrations_online()
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/migrations/env.py", line 102, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/runtime/environment.py", line 813, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/runtime/migration.py", line 560, in run_migrations
    step.migration_fn(**kw)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/airflow/migrations/versions/cf5dc11e79ad_drop_user_and_chart.py", line 49, in upgrade
    op.drop_constraint('known_event_user_id_fkey', 'known_event')
  File "<string>", line 8, in drop_constraint
  File "<string>", line 3, in drop_constraint
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/operations/ops.py", line 148, in drop_constraint
    return operations.invoke(op)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/operations/base.py", line 354, in invoke
    return fn(self, operation)
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/operations/toimpl.py", line 160, in drop_constraint
    operations.impl.drop_constraint(
  File "/Users/vijayantsoni/.virtualenvs/airflow/lib/python3.8/site-packages/alembic/ddl/sqlite.py", line 52, in drop_constraint
    raise NotImplementedError(
NotImplementedError: No support for ALTER of constraints in SQLite dialectPlease refer to the batch mode feature which allows for SQLite migrations using a copy-and-move strategy.
```
airflow/migrations/versions/cf5dc11e79ad_drop_user_and_chart.py
conn = op.get_bind()
    op.create_table(
        'users',
        sa.Column('id', sa.Integer(), nullable=False),
        sa.Column('username', sa.String(length=250), nullable=True),
        sa.Column('email', sa.String(length=500), nullable=True),
        sa.Column('password', sa.String(255)),
        sa.Column('superuser', sa.Boolean(), default=False),
        sa.PrimaryKeyConstraint('id'),
        sa.UniqueConstraint('username'),
    )
    op.create_table(
        'chart',
        sa.Column('id', sa.Integer(), nullable=False),