migrator may conflict with AutoIncrementField in sqlite
The problem is that Sqlite does not support a way to modify constraints. So in order to do this, we have to create a new table without the changed constraint, copy data into it, drop the old table, then rename the new table -> old table. In this process, your autoincrement sequence gets lost.
I have created a sqlite table 'user' using peewee:

The table generally looks like this:

> id name
> 1  name1
> 2  name2
> 3  name3
> 4  name4 (deleted)

For column id, i'm using `AutoIncrementField` from `playhouse.sqlite_ext` to prevent the deleted id from being reused

Since the name4 line has been deleted, the id of next created user should be 5

Then i used a migrator to remove the not-null constraint on field name

`migrator = SqliteMigrator(database)`
`migrator.drop_not_null('user', 'name')`

But after this i created a user and the id field was 4, the deleted id was still reused

Without `migrator.drop_not_null('user', 'name')`, the id column works as expected
