refactor: identifiers in a file (#511)

* refactor: identifiers in a file

* chore: more typing

* refactor: PlatStr -> PlatformName

* refactor: load in file, use importlib.resources

* refactor: change toml structure

* refactor: use common Traversable

* fix: ensure reasonable version of importlib_resources

* style: fix extra line

cibuildwheel/typing.py

@@ -1,6 +1,13 @@
 import os
 import subprocess
-from typing import TYPE_CHECKING, Union
+import sys
+from typing import TYPE_CHECKING, NoReturn, Set, Union
+
+if sys.version_info < (3, 8):
+    from typing_extensions import Final, Literal
+else:
+    from typing import Final, Literal
+
 
 if TYPE_CHECKING:
     PopenBytes = subprocess.Popen[bytes]
@@ -8,3 +15,11 @@ if TYPE_CHECKING:
 else:
     PopenBytes = subprocess.Popen
     PathOrStr = Union[str, "os.PathLike[str]"]
+
+
+PlatformName = Literal["linux", "macos", "windows"]
+PLATFORMS: Final[Set[PlatformName]] = {"linux", "macos", "windows"}
+
+
+def assert_never(value: NoReturn) -> NoReturn:
+    assert False, f'Unhandled value: {value} ({type(value).__name__})'  # noqa: B011