pyproject: use pdm-backend~=2.3.0.

Author: whitequark

pyproject.toml

@@ -44,7 +44,7 @@ amaranth-rpc = "amaranth.rpc:main"
 # Build system configuration
 
 [build-system]
-requires = ["pdm-backend @ git+https://github.com/pdm-project/pdm-backend@885bc8d9b75d068a34b2df86c6324c5fdf5c0cc8"]
+requires = ["pdm-backend~=2.3.0"]
 build-backend = "pdm.backend"
 
 [tool.pdm]