setuptools3: refactor for no distutils bbclasses

setuptools3: refactor for no distutils bbclasses

Add setuptools3-base.bbclass as a re-usable starting point similar to
what used to be distutils-common-base.bbclass and disutils3-base.bbclass.

We no longer need to support python2, so no need for a
setuptools-common-base.bbclass.

Refactor setuptools3.bbclass to use setuptools3-base.bbclass instead of
the distulis*.bbclasses.

Signed-off-by: Tim Orling <tim.orling@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>