classes/python_pep517: centralise shared inherits

classes/python_pep517: centralise shared inherits

For the build process to actually work some inherits are always needed:

- python3native for the python that is actually running build.
- setuptools3-base (badly named) defines how to build extensions and
  where to put modules.

Instead of inheriting these in every class, inherit them once in the
python_pep517 class.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>