Migrate to codecov-action@v4 (#358)

Author: dnerini

.github/workflows/test_pysteps.yml

@@ -100,7 +100,7 @@ jobs:
 
       - name: Upload coverage to Codecov (Linux only)
         if: matrix.os == 'ubuntu-latest'
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         env:
           OS: ${{ matrix.os }}
           PYTHON: ${{ matrix.python-version }}