Increase required flit_core version to 3.11

Needed since Flask 3.1.1 after having set the "license" keyword to an
SPDX license expression. Avoids this possible build error:

  flit_core.config.ConfigError: license field should be <class 'dict'>, not <class 'str'>

Fixes: 0109e49 ("use uv").

Author: marcows

pyproject.toml

@@ -84,7 +84,7 @@ Chat = "https://discord.gg/pallets"
 flask = "flask.cli:main"
 
 [build-system]
-requires = ["flit_core<4"]
+requires = ["flit_core>=3.11,<4"]
 build-backend = "flit_core.buildapi"
 
 [tool.flit.module]