Fix Pygments install for release check

This was accidentally added to the docutils task, and because of that
it was overriding the docutils pip install. It needs to be its own
separate task for both packages to be installed.

Change-Id: I7e5cbc34f55cd9a384468cc8c2c09a7afa2a250b

roles/check-python-release/tasks/main.yaml

@@ -8,6 +8,8 @@
     virtualenv: "{{ check_python_release_virtualenv }}"
     virtualenv_python: "{{ release_python }}"
     chdir: "{{ zuul_work_dir }}"
+
+- name: Install Pygments for the README check
   # Pygments needs to be installed to allow the metadata and README
   # check parse project READMEs that have code blocks in the
   # README.rst file.