AppVeyor: Update build environment and save artifacts (#875)

- Updates the build environment to the Visual Studio 2019 image
 - Saves the build folder as an artifact
 - Unfreeze cx_Freeze version to allow dev builds with Python 3.7 and 3.8
 - Use Python 3.8 in AppVeyor builds

appveyor.yml

@@ -1,7 +1,6 @@
-
+image: Visual Studio 2019
 environment:
-  matrix:
+  PYTHON: "C:\\Python38-x64"
-    - PYTHON: "C:\\Python36-x64"
 
 install:
   - "set PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
@@ -14,3 +13,7 @@ test_script:
 
 after_test:
   - "python setup.py build"
+  - 7z a meshroomWin64snapshot%APPVEYOR_PULL_REQUEST_HEAD_COMMIT%.zip ./build/*
+
+artifacts:
+ - path: meshroomWin64*.zip

dev_requirements.txt

@@ -1,5 +1,6 @@
 # packaging
-cx_Freeze==5.1.1
+cx_Freeze
+# use cx_Freeze==5.1.1 for Python-2
 
 # testing
 pytest