Set a default build-backend.
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,5 +1,6 @@
 [build-system]
 requires = ["setuptools>=42", "wheel", "setuptools_scm[toml]>=3.4"]
+build-backend = "setuptools.build_meta"
 
 [tool.setuptools_scm]
 write_to = "shtab/_dist_ver.py"