tmpl_render()'s "values" argument currently understands
dict[str,str|list[str]]. Enhance that to understand the broader
RenderValues type, which also includes Iterable[tuple[str, str]], as
produced by argparse.add_argument(action='append').
This commit also adds proper type-checking for the values argument.
Before, its type had gone unchecked entirely.
Signed-off-by: Jan Lindemann <jan@janware.com>