Add python-bootstrap integration for bootstrapping

When bootstrapping a new Python interpreter version, we need to bootstrap pypa/setuptools using python-bootstrap.

To reuse the same pkgbase and not run into issues in repository management later, we want to use the python-setuptools PKGBUILD to bootstrap the package with.

By setting the custom _bootstrap variable to 1 we can transparently bootstrap the package using python-bootstrap in version _bootstrap_version.

Signed-off-by: David Runge dvzrv@archlinux.org