devel/py-setuptools-scm: Fix build with Python 3.10 and py-tomli

Make this port build and run with an newer version of py-tomli by backporting
an upstream patch.

Co-authored-by: Michael Osipov <michaelo@FreeBSD.org>
PR: 293530
PR: 286779
Obtained from: https://github.com/RonnyPfannschmidt/setuptools_scm/commit/c35b53ac0dcbd0528521886612540ac6478509ee