Permit building with flit core 4 (#76)

Signed-off-by: Xu Colin <colinxu2020@gmail.com>

Author: colinxu2020

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires      = ["flit_core>=3.11,<4"]
+requires      = ["flit_core>=3.11,<5"]
 build-backend = "flit_core.buildapi"
 
 [project]