python_pep517: use picobuild instead of manually calling the API

Calling the PEP-517 API directly mostly works, but sometimes doesn't.
For example we don't verify build dependencies, which led to the cbor2
ugprade silently failing to actually package anything.

The standard frontend is pypa/build, but for source-based distributions
that can be annoying to build as it depends on the following packages:

- tomli
- pep517
- packaging
- pyparsing

Manually bootstrapping those recipes is possible, but tedious.

Picobuild is another frontend (written by myself) which is designed
explicitly to be used programatically by source-based distributions: it
doesn't support builds inside virtual environments as we're building
distribution packages, and it vendors the dependencies for bootstrapping
if they're not available.

Over time more packages are expected to move to using Flit to build
which makes the bootstrapping process slightly easier, and tomli will be
integrated into Python 3.11, so it's possible that in the future we
drop picobuild and switch to build.

This change means the PEP517_BUILD_API variable is obsolete, so remove
it.

(From OE-Core rev: 35104958181d18fd67b73943b9310c813d62b14d)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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