Permit building with flit core 4 (#76)

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

pyproject.toml

diff --git a/pyproject.toml b/pyproject.toml
index 807f358..9cce176 100644
--- a/pyproject.toml
+++ b/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]