use python 3.9

Author: isuruf

.github/workflows/multibuild.yml

@@ -51,10 +51,10 @@ jobs:
       with:
         submodules: recursive
         fetch-depth: 0
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Set extra env
       run: |
         if [ "macos-latest" == "${{ matrix.os }}" ]; then echo "TRAVIS_OS_NAME=osx" >> $GITHUB_ENV; else echo "TRAVIS_OS_NAME=${{ matrix.os }}" >> $GITHUB_ENV; fi