MNT: CI: Install types-Markdown package to avoid mypy warning

Author: kernc

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
       - name: Install lint dependencies
         if: matrix.test-type == 'lint'
         run: |
-          pip install flake8 coverage mypy
+          pip install flake8 coverage mypy types-Markdown
           sudo apt update && sudo apt-get install \
             texlive-xetex lmodern texlive-fonts-recommended  # test_pdf_pandoc
           wget -O/tmp/pandoc.deb https://github.com/jgm/pandoc/releases/download/2.10/pandoc-2.10-1-amd64.deb && sudo dpkg -i /tmp/pandoc.deb