add ytdl tests

they only run when youtube-dl or yt-dlp are installed,
i.e. if __import__("<ytdl-package>") succeeds

.github/workflows/tests.yml

@@ -26,12 +26,16 @@ jobs:
         python-version: ${{ matrix.python-version }}
 
     - name: Install dependencies
+      env:
+        PYV: ${{ matrix.python-version }}
       run: |
         pip install -r requirements.txt
+        pip install flake8
+        pip install youtube-dl
+        if [[ "$PYV" != "3.4" && "$PYV" != "3.5" ]]; then pip install yt-dlp; fi
 
     - name: Lint with flake8
       run: |
-        pip install flake8
         flake8 .
 
     - name: Run tests