Changed from flit install to pip install with opts.

This mimics normal install for testing.

.github/workflows/testing-and-coverage.yml

@@ -19,8 +19,7 @@ jobs:
     - name: install
       run: |
         python -m pip install --upgrade pip
-        pip install flit coveralls
-        flit install
+        pip install coveralls '.[test]'
     - name: run-tests
       run: |
         pytest --cov-report= --cov=emailuser tests/