load File::Slurp for black box test environment

Yes, this is needed on Python tests because black box environment is not
language specific. One script implemented in Perl runs all black box tests.

.github/workflows/test-python.yml

@@ -55,7 +55,7 @@ jobs:
         with:
           perl-version: "5.38"
           install-modules-with: cpanm
-          install-modules: Readonly File::Basename File::Slurp IPC::Run
+          install-modules: Readonly File::Basename File::Slurp IPC::Run File::Slurp
       - name: Run black box tests
         run: |
           prove test/cli-test-python.t