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

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?