Update name of source archive of BenchExec to be compatible with PEP 625

[PhilippWendler]

Currently our release script produces files named like BenchExec-3.27.tar.gz, which are incompatible with PEP 625. PyPI warns that they are deprecated and future uploads need to conform to the new naming scheme. Likely we need to update setuptools, which we pinned to an older version in 57f5f40 exactly in order to keep the old names of our release artifacts. Then we probably also need to adjust our release script.