Relax build-system requirement

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["flit_core >=3.2.0,<3.3"]
+requires = ["flit_core >=3.2.0,<4"]
 build-backend = "flit_core.buildapi"
 
 [project]