python-schema: drop contextlib2 dependency

python-schema: drop contextlib2 dependency

Upstream only requires contextlib2 on Python <3.3, so it is never
pulled in on a Python 3.14 runtime.

schema builds through the setuptools.build_meta legacy backend but never
declared setuptools as a build dependency; it was only present in the host
build env transitively via contextlib2's build. Add python-setuptools/host
explicitly so the build no longer relies on that side effect.

Signed-off-by: Alexandru Ardelean <redacted>