ci: make a separate job for failure notifiications

Author: zimeg

.github/workflows/tests.yml

@@ -81,6 +81,11 @@ jobs:
           flags: ${{ matrix.python-version }}
           token: ${{ secrets.CODECOV_TOKEN }}
           verbose: true
+  notifications:
+    runs-on: ubuntu-latest
+    needs: build
+    if: failure()
+    steps:
       - name: Send notifications of failing tests
         if: failure()
         uses: slackapi/slack-github-action@b0fa283ad8fea605de13dc3f449259339835fc52 # v2.1.0