pex: fix python3-pep517 build

srcpkgs/pex/patches/flit_version.patch

@@ -0,0 +1,13 @@
+The upper restriction on the flit_core version appears to be a stock
+generational restriction rather than a specific requirement, and the package
+seems to build as expected anyway. Drop the requirement restriction.
+
+--- a/pyproject.toml
++++ b/pyproject.toml
+@@ -1,5 +1,5 @@
+ [build-system]
+-requires = ["flit_core >=2,<3"]
++requires = ["flit_core"]
+ build-backend = "flit_core.buildapi"
+ 
+ [tool.flit.metadata]