Use older pip version to remain compatible with Python 3.7

.github/workflows/release.yml

@@ -112,7 +112,7 @@ jobs:
  --slave /usr/bin/g++ g++ /usr/bin/g++-9
  ln -sf cpp-9 /usr/bin/cpp
  update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1
- curl -s https://bootstrap.pypa.io/get-pip.py -o get-pip.py
+ curl -s https://bootstrap.pypa.io/pip/3.7/get-pip.py -o get-pip.py
  python3 get-pip.py --force-reinstall
  rm get-pip.py