pin coverage to version 6.2 to try to stop pool hanging in pytest (#60)

.github/workflows/build.yml

@@ -29,6 +29,7 @@ jobs:
         pip install flake8
         pip install -r requirements.txt
         pip install -r companion.txt
+        pip install "coverage==6.2"
         pip install coveralls
         pip install .
     - name: Lint with flake8