pypy27Packages.incremental: fix the eval

Without the change the eval fails as:

    $ nix-instantiate -A pypy27Packages.incremental
    error:
       … while evaluating the attribute 'drvPath'
         at lib/customisation.nix:365:7:
          364|     in commonAttrs // {
          365|       drvPath = assert condition; drv.drvPath;
             |       ^
          366|       outPath = assert condition; drv.outPath;

       … while evaluating the attribute 'drvPath'
         at lib/customisation.nix:365:7:
          364|     in commonAttrs // {
          365|       drvPath = assert condition; drv.drvPath;
             |       ^
          366|       outPath = assert condition; drv.outPath;

       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:34:12:
           33|
           34|   strict = derivationStrict drvAttrs;
             |            ^
           35|

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: cannot coerce a set to a string: { scripts = «thunk»; }

NixOS#335112 (comment)
suggests that "The Python 2 version of buildPythonPackage doesn't support
optional-dependencies outside of passthru."

Added `passthru` explicitly.

pkgs/development/python-modules/incremental/default.nix

@@ -28,7 +28,7 @@ let
 
     dependencies = [ setuptools ] ++ lib.optionals (pythonOlder "3.11") [ tomli ];
 
-    optional-dependencies = {
+    passthru.optional-dependencies = {
       scripts = [ click ];
     };