CI: Fix tests on pypy3 by installing libxml2-dev and libxslt-dev

Author: amotl

.github/workflows/test.yml

@@ -25,6 +25,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
       - name: Install dependencies
         run: |
+          sudo apt install libxml2-dev libxslt-dev
           python -m pip install --upgrade pip
           python -m pip install .[test]
         shell: bash