[Python-Dev] Generate all Argument Clinic code into separate files

[Serhiy Storchaka]

Currently for some files converted to use Argument Clinic the generated 
code is written into a separate file, for other files it is inlined.

I'm going to make a number of small enhancement to Argument Clinic to 
generate faster code for parsing arguments (see for example issue23492). 
Every such step will produce large diffs for
generated code and will create code churn if it is inlined and mixed up 
with handwritten code. It would be better when only generated files will 
be changed. So I suggest to move all inlined generated code in separate 
file. What are you think?