Run CI for Windows

As a precursor to #143

Author: chrisjsewell

.github/workflows/tests.yml

@@ -6,10 +6,14 @@ jobs:
 
   tests:
 
-    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.5, 3.6, 3.7, 3.8]
+        include:
+          - os: windows-latest
+            python-version: 3.7
+    
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v2