use code generation for scoped_session

use code generation for scoped_session

our decorator thing generates code in any case,
so point it at the file itself to generate real code
for the blocks rather than doing things dynamically.

this will allow typing tools to have no problem
whatsoever and we also reduce import time overhead.
file size will be a lot bigger though, shrugs.

syntax / dupe method / etc. checking will be accomplished
by our existing linting / typing / formatting tools.

As we are also using "from __future__ import annotations",
we also no longer have to apply quotes to generated
annotations.

Change-Id: I20962cb65bda63ff0fb67357ab346e9b1ef4f108