When fixing some bugs (#118814, #122688) I noticed also design problems in _PyArg_UnpackKeywordsWithVararg().

• Its vararg parameter is always equal to maxpos, therefore it is redundant.
• Inserting a new tuple between values for positional and keyword-only parameters adds complexity in the _PyArg_UnpackKeywordsWithVararg() code which caused bugs.
• Since it is the only argument value which is a strong reference, it adds complexity for cleanup after calling _PyArg_UnpackKeywordsWithVararg().
• This large code is mostly a duplication of _PyArg_UnpackKeywords().
• But it lacks some microoptimizations.
• And produces wrong error messages in some corner cases.
• Also some cases (like var-positional parameter after optional parameters) were not supported.

So I re-worked it in several steps:

• Removed the vararg parameter from _PyArg_UnpackKeywordsWithVararg().
• Moved creation of the tuple for var-positional parameter out from _PyArg_UnpackKeywordsWithVararg().
• Refactored Argument Clinic code: it now generates similar code for var-positional parameter in functions with and without keyword parameters. The generated code is now more optimal and more cases are now supported. This is a large step.
• Finally, _PyArg_UnpackKeywordsWithVararg() was merged with _PyArg_UnpackKeywords() which now got a new flag.

Linked PRs

• gh-122943: Rework support of var-positional parameter in Argument Clinic #122945
• gh-122943: Remove the object converter for var-positional parameter #126560
• gh-122943: Add the varpos parameter in _PyArg_UnpackKeywords #126564
• gh-122943: Move code generation for var-positional parameter to converters #126575