Add missing build-system section.
Backported from https://github.com/scott-griffiths/bitstring/commit/1d02d003d6bbb81a7a26d64bc970b57770944523

--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,3 +1,6 @@
+[build-system]
+requires = ["setuptools>=61"]
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "bitstring"