Install zsh in GHA

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index dbfbbea..b268257 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -24,6 +24,7 @@
 
     - name: Install packages
       run: |
+        sudo apt-get install --yes zsh
         export PYTHONIOENCODING=UTF8
         pip install coveralls pytest-cov ptyprocess