Try fix appveyor build

Author: bakwc

appveyor.yml

@@ -8,6 +8,7 @@ environment:
     - PYTHON: "C:\\Python38"
 
 install:
+  - "%PYTHON%\\python.exe -m pip install --upgrade pip"
   - "%PYTHON%\\python.exe -m pip install pytest"
   - "%PYTHON%\\python.exe -m pip install cryptography"