Build-Depends on python3-setuptools-scm to get correct Python version (Closes: #1005042)
Following the recent discussion on the debian-python list (see https://lists.debian.org/debian-python/2022/02/msg00024.html), this fixes https://bugs.debian.org/1005042 by reinstating the use of setuptools-scm and ensuring that the _version.py file is removed after the cleaning phase.