incompatibility with matplotlib 3.7

[edublancas]

Our tests started failing with matplotlib 3.7, I tracked down the error locally:

# ensure you're running matplotlib 3.7
pip install matplotlib -U

# run tests
pytest tests --ignore=tests/test_engine.py

the error is happening in the test_no_outputs test. Upon execution, this is how the notebook looks like:

there's a clear error here since the first cell x = 1 should not produce any outputs; yet, it displays a plot. Seems like with the new matplotlib release, we need to do some "matplotlib state cleaning" when destroying the shell object, so when creating a new one, we don't share state from previous notebook runs. My guess is that we're missing something here

It's still unclear what changed in matplotlib 3.7 so more investigation is needed. for the time being, I pinned matplotlib so our tests pass.