Add appveyor build for Py3.8 (will probably fail tests, but it should at least build).

appveyor.yml

@@ -17,6 +17,14 @@ environment:
       PYTHON_VERSION: "2.7"
       PYTHON_ARCH: "64"
 
+    - PYTHON: "C:\\Python38"
+      PYTHON_VERSION: "3.8"
+      PYTHON_ARCH: "32"
+
+    - PYTHON: "C:\\Python38-x64"
+      PYTHON_VERSION: "3.8"
+      PYTHON_ARCH: "64"
+
     - PYTHON: "C:\\Python37"
       PYTHON_VERSION: "3.7"
       PYTHON_ARCH: "32"