CI: Use "windows-2019" to improve runtime

Author: amotl

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest, windows-2019]
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10']
 
     steps: