python_pep517: use python3-build, not picobuild

Now that the bootstrap sequence for pypa/build is just
python3-packaging, and python3-pyproject-hooks, we can use build instead
of picobuild.

The only change visible outside of this class is that the variable
PEP517_PICOBUILD_OPTS is now PEP517_BUILD_OPTS, but I'm only aware of one
recipe that used that variable.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>