Update Windows CI job

We now use `choco` instead of `nuget`, and invoke the proper Python executable with the `py` launcher.

.gitlab-ci.yml

@@ -13,34 +13,22 @@ windows job:
     - windows
 
     before_script:
-    - pushd $HOME
-    - mkdir pythons
-    - cd pythons
-    - curl https://aka.ms/nugetclidl -o nuget.exe
-
-    - .\nuget.exe install python -Version 3.6.8
-    - move python.3.6.8\tools\python.exe python.3.6.8\tools\python3.6.exe
-    - $Env:Path+=";$HOME\pythons\python.3.6.8\tools"
-    - python3.6 -m pip install --upgrade pip
-
-    - .\nuget.exe install python -Version 3.7.7
-    - move python.3.7.7\tools\python.exe python.3.7.7\tools\python3.7.exe
-    - $Env:Path+=";$HOME\pythons\python.3.7.7\tools"
-    - python3.7 -m pip install --upgrade pip
-
-    - .\nuget.exe install python -Version 3.8.3
-    - move python.3.8.3\tools\python.exe python.3.8.3\tools\python3.8.exe
-    - $Env:Path+=";$HOME\pythons\python.3.8.3\tools"
-    - python3.8 -m pip install --upgrade pip
-
-    - .\nuget.exe install python -Version 3.9.0-b4
-    - move python.3.9.0-b4\tools\python.exe python.3.9.0-b4\tools\python3.9.exe
-    - $Env:Path+=";$HOME\pythons\python.3.9.0-b4\tools"
-    - python3.9 -m pip install --upgrade pip
-
-    - python3.8 -m pip install tox
-    - $Env:Path+=";$HOME\pythons\python.3.8.3\tools\Scripts"
-    - popd
+    # Allow chocolatey to install without confirmation.
+    - choco feature enable -n allowGlobalConfirmation
+    # Install latest stable Python (currently 3.9).
+    - choco install python -r
+    # Add default Python and its scripts to the Path.
+    - $ENV:Path+=";C:\Python39;C:\Python39\Scripts"
+    # Install older Pythons.
+    - choco install python -r --side-by-side --version=3.8.6
+    - choco install python -r --side-by-side --version=3.7.9
+    - choco install python -r --side-by-side --version=3.6.8
+    # Upgrade/install the latest pip and tox.
+    - py -m pip install --upgrade pip
+    - py -3.8 -m pip install --upgrade pip
+    - py -3.7 -m pip install --upgrade pip
+    - py -3.6 -m pip install --upgrade pip
+    - py -m pip install tox
 
     script:
     - tox