Update dependency flit_core to v4

Author: renovate[bot]

pyproject.toml

@@ -55,7 +55,7 @@ Source = "https://github.com/marshmallow-code/webargs"
 Tidelift = "https://tidelift.com/subscription/pkg/pypi-webargs?utm_source=pypi-webargs&utm_medium=pypi"
 
 [build-system]
-requires = ["flit_core<4"]
+requires = ["flit_core<5"]
 build-backend = "flit_core.buildapi"
 
 [dependency-groups]