upgrade pip?

.github/workflows/build.yml

@@ -39,6 +39,8 @@ jobs:
           python --version
           file "$(which python)"
 
+          python -m pip install --upgrade pip
+
           # wxPython doen't publish linux wheels in pypi
           wget -q https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-16.04/wxPython-4.0.6-cp27-cp27mu-linux_x86_64.whl
           python -m pip install wxPython*.whl