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In progress. Actually many functions are not very suitable for Argument Clinic.

What functions, and what makes them unsuitable?

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.

(It would have made it easier on me if you'd used the C function names, instead of the names in the module.)
I agree. In retrospect, it's not surprising that things in _tkinter aren't suitable, as it appears to be a shunt for calling things in Tcl.
For what it's worth, Argument Clinic is only intended for functions that use PyArg_ParseTuple or PyArg_ParseTupleOrKeywords to parse their arguments. So something like call is and will forever be unsuitable.
(And reading _tkinter.c this has reminded me why I don't like Tcl ;-)

So do you think there are any entry points worth converting in _tkinter, or should we close this issue?

You misunderstood me. I already have 1018-lines patch which converts about 24 
functions and methods to Argument Clinic church. And I'm working on left 
functions. Do you want to get unfinished patch?

Oh, okay. That sounds fine, and I'm not in a hurry. Please post the patch whenever you're ready, thanks!

Here is a patch.

Here is updated patch.

Synchronized with tip.

Updated to the tip. Converted methods getint, getdouble, getboolean, splitlist, split.

New changeset b22ced894d51 by Serhiy Storchaka in branch 'default':
Issue #20168: Converted the _tkinter module to Argument Clinic.
https://hg.python.org/cpython/rev/b22ced894d51 

wantobjects() was not converted due to a bug in Argument Clinic (issue24051).

This broke Windows builds because of unnecessary "static" qualifiers on the forward declarations at lines 2685 and 3006 (as discussed on #20323).
Removing "static" from these lines fixes the build.

As mentioned on #20148, removing the "static" from the forward declarations breaks it on GCC. (And, of the two, I think GCC is the one being reasonable here.)
Attached is a patch that should fix the build for tkinter.

LGTM

New changeset 7a76c462c7f6 by Larry Hastings in branch 'default':
Fix Windows build breakage from checkins on Issues #20148 and #20168.
https://hg.python.org/cpython/rev/7a76c462c7f6 

Steve, please close this issue when you've confirmed it's now building correctly on Windows.