Fix setup-python installation error with 3.5

See actions/setup-python#866
actions/setup-python#866 (comment)

Author: imjoehaines

.github/workflows/python-package.yml

@@ -14,6 +14,7 @@ jobs:
         include:
           - python-version: '3.5'
             os: 'ubuntu-20.04'
+            pip-trusted-host: 'pypi.python.org pypi.org files.pythonhosted.org'
           - python-version: '3.6'
             os: 'ubuntu-20.04'
 
@@ -24,6 +25,8 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
+      env:
+        PIP_TRUSTED_HOST: ${{ matrix.pip-trusted-host }}
 
     - name: Install dependencies
       run: |