text: Title: Database autogenerate migrations
  Body: * GINO version: 0.8.3
  * Python version: 3.7.5
  * asyncpg version: 0.19.0
  * aiocontextvars version: 0.2.2
  * PostgreSQL version: 10.10
  
  ### Description
  How to make model's `__tablename__` is autogenerated? 
  I've tried to using `gino.declarative.declared_attr` to automatically generate `__tablename__` attribute for model, but got error
  
  `KeyError: '<function BaseModel.__tablename__ at 0x7f21defb7950>'`
  
  ### What I Did
  I tried to structuring applications with `starlette` here https://github.com/nsiregar/letsgo but unable to autogenerate `__tablename__`
  