level up pep-484 on engine result

Have each result subclass be generic to the
kind of row/object it returns.  rework things so that
a significant number of "type ignores" can go away
and also allow asyncio to more cleanly proxy
the result objects.

Change-Id: Ia3ddb2cb52f5856839bd8c9c46f0289ab4b10405

Author: zzzeek

lib/sqlalchemy/engine/base.py

@@ -1150,7 +1150,7 @@ def scalars(
         statement: Executable,
         parameters: Optional[_CoreSingleExecuteParams] = None,
         execution_options: Optional[_ExecuteOptionsParameter] = None,
-    ) -> ScalarResult:
+    ) -> ScalarResult[Any]:
         """Executes and returns a scalar result set, which yields scalar values
         from the first column of each row.