python3 absolute import error : I meet a problem in python3 absolute import, here is my project structure:
`
-project_dir
 -src
  __init__.py
  index.py
  -handler
   __init__.py
   base_handler.py
`

in `index.py` I import `base_handler.BaseHandler` like this:
`from src.handler.base_handler import BaseHandler`

and an error occurs:
`ModuleNotFoundError: No module named 'src'`

The strange thing is, when I move `index.py` out of `src` folder, this error whill disappear:
`
-project_dir
 index.py
 -src
  __init__.py
  -handler
   __init__.py
   base_handler.py
`
Now in `index.py` I can import `BaseHanlder` like this:
`from src.handler.base_handler import BaseHandler`

anyone can tell me why this happens? thx
