Permit building with flit core 4

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

Author: colinxu2020

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ['flit_core >=3.2,<4']
+requires = ['flit_core >=3.2,<5']
 build-backend = 'flit_core.buildapi'
 
 [project]