python3: don't use sysconfigdataHook on darwin

Resolves issue when building wheels which is a regression introduced by
1a65c5df5f

```
AssertionError: would build wheel with unsupported tag ('cp38', 'cp38',
'darwin_x86_64')
```

pkgs/development/interpreters/python/cpython/default.nix

@@ -370,7 +370,7 @@ in with passthru; stdenv.mkDerivation {
 
   # Add CPython specific setup-hook that configures distutils.sysconfig to
   # always load sysconfigdata from host Python.
-  postFixup = ''
+  postFixup = stdenv.lib.optionalString (!stdenv.hostPlatform.isDarwin) ''
     cat << "EOF" >> "$out/nix-support/setup-hook"
     ${sysconfigdataHook}
     EOF