call() just converts all args to Tcl list. Is Argument Clinic supports *args 
and **kwargs? I'm not sure there is a benefit with using Argument Clinic here.

splitlist() and split() first call PyArg_ParseTuple with one argument, check 
the input, and then may call PyArg_ParseTuple with other argument. They need 
large refactoring to use Argument Clinic.

getint(), getdouble(), getboolean() first manually unpack arguments tuple, and 
then may call PyArg_ParseTuple. Same as above.

setvar()/globalsetvar(), getvar()/globalgetvar(), unsetvar()/globalunsetvar() 
shares common code parametrized by flags. I think this is resolvable. But 
worse, they can delegates their execution to other thread and should save all 
arguments in events queue. And this looks absolutely unsuitable for Argument 
Clinic.