replace .ext. with _

[kigawas]

gino.ext.starlette should be replaced since lint tools work poorly with it

[fantix]

Oh, for the linter/typing to work nicely, pls consider running:

$ python -m gino.ext stub

Refs python-gino/gino#674.

[kigawas]

I remember the days I was using Flask.

At that time, the extensions were named like flask.ext.sqlalchemy to mitigate the pain of package transitions but finally it got deprecated and removed.

Do we still need an ext stuff in 2020? We have lots of package management tools like poetry or flit, it won't be a pain for users to update the package, even gino_starlette becomes like starlette_gino or something else.