python3-pytest: clean up RDEPENDS

python3-pytest: clean up RDEPENDS

These dependencies are only used in development:
- attrs
- setuptools

These dependencies are obsolete:
- atomicwrites removed 7.2.0
- importlib-metadata removed in 5.1.0
- more-itertools removed 6.1.0
- pathlib2 removed 6.2.2
- py removed in 7.2
- six removed in 5.0.0
- wcwidth removed in 6.0.0

This removals now means that python3-xml has to be added explicitly.

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