ci: explicitly upgrade pdm

Author: waketzheng

.github/workflows/ci.yml

@@ -128,7 +128,8 @@ jobs:
           curl https://bootstrap.pypa.io/pip/3.7/get-pip.py -o get-pip.py
           python3 get-pip.py
           python3 -m pip install pip --upgrade
-          python3 -m pip install pdm
+          python3 -m pip install pdm --upgrade
+          pdm config python.use_freethreaded false
       - name: setup poetry
         run: |
           curl https://bootstrap.pypa.io/get-pip.py -o get_pip.py