Non-determinism of mode for a plotly express line plot with markers

[alev000]

The following block of code introduces nondeterminism in the mode of a plotly express trace, because it involves iterating over a set:
https://github.com/plotly/plotly.py/blob/v5.18.0/packages/python/plotly/plotly/express/_core.py#L1922-L1931

Among other things, this causes the output of plotly.graph_objects.Figure.to_html to be non-deterministic, because sometimes the mode for a line plot with markers is "lines+markers" and other times it is "markers+lines". This makes it difficult to test for consistency of results during automated tests.

One potential solution might be to replace the following:

-        trace_patch["mode"] = "+".join(modes)
+        trace_patch["mode"] = "+".join(sorted(modes))

Alternatively, modes could be constructed as a list instead of a set.

[alexcjohnson]

Nice find @alev000 - seems like a good solution, feel free to make a PR