update appveyor for cpython3.8 release

Author: mattip

.appveyor.yml

@@ -76,9 +76,10 @@ test_script:
     - python -m pyappveyordemo.tests.test_extension
 
     # Smoke test of install_python script
+    # Use C:\PythonXY, C:\PythonXY-x64, C:\PythonXYrcZ, or C:\PythonXYrcZ-x64
     - set PYTHON=C:\Python37
     - ps: .\install_python.ps1
     - set PYTHON=C:\Python37-x64
     - ps: .\install_python.ps1
-    - set PYTHON=C:\Python38rc1-x64
+    - set PYTHON=C:\Python38-x64
     - ps: .\install_python.ps1