Install six in CI

Workarounds transifex/transifex-python#121. Closes #46.

Author: m-aciek

.github/workflows/update-lint-and-build.yml

@@ -19,7 +19,7 @@ jobs:
         with:
           python-version: 3
       - run: sudo apt-get install -y gettext
-      - run: pip install requests cogapp polib transifex-python sphinx-intl blurb
+      - run: pip install requests cogapp polib transifex-python sphinx-intl blurb six
       - run: curl -o- https://raw.githubusercontent.com/transifex/cli/master/install.sh | bash
         working-directory: /usr/local/bin
       - uses: actions/checkout@master