build: Add the missing build-system section to fix building (#100)

Add the missing `build-system` section to fix compatibility with strict
PEP 517 installers.  Per PEP 517, projects are required to either
specify a backend explicitly *or* (for compatibility with legacy
projects) provide a `setup.py` file.

Author: mgorny

pyproject.toml

@@ -1,3 +1,7 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
+
 [project]
 name = "pypi-attestations"
 dynamic = ["version"]