⬆️ Allow flit_core 4 as build backend (#1185)

Update the build requirement from flit_core>=3.4,<4 to >=3.4,<5.

Author: dependabot[bot]

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["flit_core >=3.4,<4"]
+requires = ["flit_core >=3.4,<5"]
 build-backend = "flit_core.buildapi"
 
 [project]