CI: bump python version.

Author: tasn

.github/workflows/manual.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/setup-python@v2
         name: Install Python
         with:
-          python-version: '3.7'
+          python-version: '3.8'
 
       - name: Update pip and install deps
         run: |