appveyor: Use Python3.

This implied upgrading to the Visual Studio 2019 image, not for VS
itself, but for the newer Python 3.8.5 version it contains, to avoid
UnicodeDecodeError inside modulefinder module when attempting to decode
our UTF-8 encoded Python scripts with cp1252 encoding.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6184>

.appveyor/appveyor_msvc.bat

@@ -7,13 +7,13 @@ python -m pip install --upgrade pip
 python -m pip --version
 if "%buildsystem%" == "scons" (
     rem Install Mako
-    python -m pip install Mako==1.0.7
+    python -m pip install Mako==1.1.3
     rem Install pywin32 extensions, needed by SCons
     python -m pip install pypiwin32
     rem Install python wheels, necessary to install SCons via pip
     python -m pip install wheel
     rem Install SCons
-    python -m pip install scons==3.0.1
+    python -m pip install scons==3.1.2
     call scons --version
 ) else (
     python -m pip install Mako meson
@@ -44,7 +44,7 @@ goto :eof
 
 :build_script
 if "%buildsystem%" == "scons" (
-    call scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.1 llvm=1
+    call scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.2 machine=x86 llvm=1
 ) else (
     call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\Tools\VsDevCmd.bat" -arch=x86
     rem We use default-library as static to affect any wraps (such as expat and zlib)
@@ -59,7 +59,7 @@ goto :eof
 
 :test_script
 if "%buildsystem%" == "scons" (
-    call scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.1 llvm=1 check
+    call scons -j%NUMBER_OF_PROCESSORS% MSVC_VERSION=14.2 machine=x86 llvm=1 check
 ) else (
     call meson test -C builddir
 )

appveyor.yml

@@ -40,7 +40,7 @@ cache:
 - llvm-5.0.1-msvc2017-mtd.7z
 - subprojects\packagecache -> subprojects\*.wrap
 
-os: Visual Studio 2017
+os: Visual Studio 2019
 
 init:
 # Appveyor defaults core.autocrlf to input instead of the default (true), but
@@ -48,6 +48,7 @@ init:
 - git config --global core.autocrlf true
 
 environment:
+  Path: C:\Python38-x64;C:\Python38-x64\Scripts;%Path%
   WINFLEXBISON_VERSION: 2.5.15
   LLVM_ARCHIVE: llvm-5.0.1-msvc2017-mtd.7z
   matrix: