Remove Python 3.8 from AppVeyor since it doesn't seem to yet support it, at least not in a Miniconda environment

Author: tleonhardt

.appveyor.yml

@@ -8,14 +8,11 @@ environment:
       PYTHON_VERSION: "3.6.x"
       PYTHON_ARCH: "64"
       TOXENV: "py36"
+
     - PYTHON: "C:\\Miniconda37-x64"
       PYTHON_VERSION: "3.7.x"
       PYTHON_ARCH: "64"
       TOXENV: "py37"
-    - PYTHON: "C:\\Miniconda38-x64"
-      PYTHON_VERSION: "3.8.x"
-      PYTHON_ARCH: "64"
-      TOXENV: "py38"
 
 init:
   - "%PYTHON%/python -V"