Fix unit tests

Author: timvink

.github/workflows/unittests_codecov.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.7, 3.8, 3.9, 3.10]
+        python-version: [3.7, 3.8, 3.9, "3.10"]
     steps:
     - uses: actions/checkout@master