pythonInterpreters.pypy39_prebuilt: fix `tests` eval (#278950)

Without the change `test-pythonPackagesExtensions` test fails the eval
as:

    $ nix build -f. pythonInterpreters.pypy39_prebuilt.tests --show-trace
    error:
       … while evaluating an attribute name
         at pkgs/development/interpreters/python/tests.nix:151:16:
          150|       });
          151|     in pkgs_.${python.pythonAttr}.pkgs.foo;
             |                ^
          152|   });

       error: value is null while a string was expected

This happens because `pypy39_prebuilt` exposes `pythonAttr` attribute,
but it has a `null` value. Fix the test to filter out prebuilt pypy.

0 files changed, 0 insertions, 0 deletions