CI: Appveyor Python 3.7->3.8

Update install. Potentially move to GH actions as a follow-up.

Author: ax3l

.appveyor.yml

@@ -1,15 +1,15 @@
 environment:
   matrix:
     - TARGET_ARCH: x64
-      CONDA_PY: 3.7
-      CONDA_INSTALL_LOCN: C:\\Miniconda37-x64
+      CONDA_PY: 3.8
+      CONDA_INSTALL_LOCN: C:\\Miniconda38-x64
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       platform: x64
       SHARED: OFF
 
     - TARGET_ARCH: x86
-      CONDA_PY: 3.7
-      CONDA_INSTALL_LOCN: C:\\Miniconda37
+      CONDA_PY: 3.8
+      CONDA_INSTALL_LOCN: C:\\Miniconda38
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       platform: x86
       SHARED: ON