repair type qualify in _ServerDefaultType; other pyright tweaks

repair type qualify in _ServerDefaultType; other pyright tweaks

as we haven't done full pylance / pyright strict typing internally,
some of the things pyright reports on specifically will leak
out into user code, such as this mapped_column() issue.
So we will have to look more closely at pyright strict
mode going forward for the release.

Fixed typing issue where pylance strict mode would report "partially
unknown" datatype for the :func:`_orm.mapped_column` construct.

Also repaired a trailing comma and pyright complaining about overloads
for orm.composite.

Fixes: #8644
Change-Id: Ia48dc5dbd56bbceeacee4f0daf9810bfdea3bee3