Appveyor: Quickly fix meson build.

As this required use of Python 3.8, mako module also had to be updated.

v2 - Unbind mako module version when using Meson.
Signed-off-by: Prodea Alexandru-Liviu <liviuprodea@yahoo.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

.appveyor/appveyor_msvc.bat

@@ -16,7 +16,7 @@ if "%buildsystem%" == "scons" (
     call scons --version
 ) else (
     python --version
-    python -m pip install Mako==1.0.7 meson
+    python -m pip install Mako meson
     meson --version
 
     rem Install pkg-config, which meson requires even on windows

appveyor.yml

@@ -55,7 +55,7 @@ environment:
     buildsystem: scons
   - compiler: msvc
     buildsystem: meson
-    path: C:\Python37-x64;C:\Python37-x64\Scripts;%path%
+    path: C:\Python38-x64;C:\Python38-x64\Scripts;%path%
 
 install:
 - cmd: .appveyor\appveyor_msvc.bat install