It seems there are no ubuntu-18.04 runners any more in GH actions. Le…

…t's see if the tests run on Mac OS, Windows.

Reduce the number of Python versions tested to keep the total test matrix size reasonable.

.github/workflows/tests.yml

@@ -7,15 +7,8 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-latest]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
-        include:
-          - os: "ubuntu-18.04"
-            python-version: "3.4"
-          - os: "ubuntu-18.04"
-            python-version: "3.5"
-          - os: "ubuntu-18.04"
-            python-version: "3.6"
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: ["3.7", "3.9", "3.11", "3.12"]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3