Upgrade to (non-bash) Codecov v2 Coverage Report Uploader

Author: amotl

.github/workflows/tests.yml

@@ -58,9 +58,9 @@ jobs:
 
     - name: Upload coverage to Codecov
       if: matrix.os == 'ubuntu-latest' && matrix.python-version == '3.9'
-      uses: codecov/codecov-action@v1
+      uses: codecov/codecov-action@v2
       with:
-        file: ./coverage.xml
+        files: ./coverage.xml
         flags: unittests
         env_vars: OS,PYTHON
         name: codecov-umbrella