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

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

To reuse the same pkgbase and not run into issues in repository
management later, we want to use the python-packaging 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>

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