text: Title: error: Unexpected keyword argument "astext_type" for "JSONB"
  Body: I'm getting `error: Unexpected keyword argument "astext_type" for "JSONB"` in my alembic migrations on lines like:
  `sa.Column("data", postgresql.JSONB(astext_type=sa.Text()), nullable=True),`
  
  Looking at the [__init__](https://github.com/sqlalchemy/sqlalchemy/blob/rel_1_4_0b1/lib/sqlalchemy/dialects/postgresql/json.py#L183) function of the JSON class that JSONB inherits it has a keyword argument astext_type.
  
  Can't find the __init__ function for JSON/JSONB in this repo. 
  
  So, am I doing something wrong or is there something not implemented or whatnot?
  
  Thanks!