ci: skip windows python 3.6 environment

This is done so since github's windows environment cannot reliably
compile datetime backports.

Author: abn

.github/workflows/ci.yml

@@ -16,7 +16,9 @@ jobs:
       matrix:
         os: [Ubuntu, MacOS, Windows]
         python-version: [3.6, 3.7, 3.8]
-
+        exclude:
+          - os: Windows
+            python-version: 3.6
     steps:
       - uses: actions/checkout@v2