ci: Launch tmate session if pytest fails on workflow dispatch run (#1748

)

* If the pytest jobs fail and the ci workflow was triggered as the result of
a workflow dispatch, launch a tmate session which allows for sshing into the
runner with the environment from the failed job.
   - c.f. https://github.com/mxschmitt/action-tmate

This idea comes from Simon Willison's (@simonw) blog post:
https://til.simonwillison.net/github-actions/debug-tmate

.github/workflows/ci.yml

@@ -51,6 +51,10 @@ jobs:
       run: |
         pytest -r sx --ignore tests/benchmarks/ --ignore tests/contrib --ignore tests/test_notebooks.py
 
+    - name: Launch a tmate session if tests fail
+      if: failure() && github.event_name == 'workflow_dispatch'
+      uses: mxschmitt/action-tmate@v3
+
     - name: Report core project coverage with Codecov
       if: github.event_name != 'schedule' && matrix.python-version == '3.9' && matrix.os == 'ubuntu-latest'
       uses: codecov/codecov-action@v2