fix: A type alias cannot be final (#1024)

Addition of Python 3.10 to the checks exposed this.

Author: henryiii

cibuildwheel/util.py

@@ -45,7 +45,7 @@
 
 install_certifi_script: Final = resources_dir / "install_certifi.py"
 
-BuildFrontend: Final = Literal["pip", "build"]
+BuildFrontend = Literal["pip", "build"]
 
 MANYLINUX_ARCHS: Final = (
     "x86_64",