Deterministic sort order for plotly express modes

[alev000]

Addresses #4428

Note: without the proposed code change to _core.py, the new tests fail intermittently; with the proposed change, they succeed consistently.

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
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  new tutorial notebook (please see the doc checklist as well).
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• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[alexcjohnson]

💃 Very nice test, thanks! Let's add a changelog entry then we can merge.

[alev000]

I see a failure in python_38_orca here. From what I can see, this seems unrelated to the current PR. Is this potentially a flaky test failure, or a transient error that needs to be retried?

plotly/tests/test_orca/test_orca_server.py::test_ensure_orca_ping_and_proc

[alexcjohnson]

Yep, just a flaky test - all good the second time around ☺️ - thanks again!