allow any key for naming_convention dict, typing is not possible

allow any key for naming_convention dict, typing is not possible

Fixed regression introduced in 2.0.20 via :ticket:`9600` fix which
attempted to add more formal typing to
:paramref:`_schema.MetaData.naming_convention`. This change prevented basic
naming convention dictionaries from passing typing and has been adjusted so
that a plain dictionary of strings for keys as well as dictionaries that
use constraint types as keys or a mix of both, are again accepted.

As part of this change, lesser used forms of the naming convention
dictionary are also typed, including that it currently allows for
``Constraint`` type objects as keys as well.

Fixes: #9284
Fixes: #10264
Change-Id: Ic6561dd65058e4de3a7a393295b9863fc065db13