better python_pep517

[nekopsykose]

• run tests in some other cwd than build_wrksrc (to avoid import mod picking ./mod instead of built native mod in site-packages which breaks some testsuites currently, also see e.g. numpy)
• put stuff in util/ in case needed to reuse for stuff to call build/install (e.g. certbot)
• respect common stuff like make_check_wrapper
• (?) integrate some plugins when installed like xdist and maybe something else so --numprocesses/--loadfile etc don't have to be manually passed
• maybe more better pytest handling..

[triallax]

not sure about this one, but maybe also integration with python-setuptools_scm (i.e. setting SETUPTOOLS_SCM_PRETEND_VERSION)?