recipetool/create_buildsys_python: refactor code for futur PEP517 addition

recipetool/create_buildsys_python: refactor code for futur PEP517 addition

In order to prepare the support for pyproject.toml (PEP517 [1]) enabled
projects, refactor the code and move setup.py specific code into a
specific class in order to allow sharing the PythonRecipeHandler class

No functionnal changes expected

[1]: https://peps.python.org/pep-0517/#source-tree

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>