feat(python): Add mermaid output to show_graph

[kjgoodrick]

This is a fix for #12075 adding mermaid output to polars. I have decided that it makes more sense to have mermaid output come from the show_graph method instead of explain because explain appears to be for generating printable representations of the query plan wheras show_graph is for generating strings that can be fed to a visualization engine (currently just graphviz)

In addition to being able to output mermaid strings this PR will output a mermaid diagram if the user is in a notebook and graphviz is not installed. I think at some point it may be a better user experience to always export mermaid instead of the SVG in notebooks, but for now I did not want to change the behavior.

[codecov]

Codecov Report

Attention: Patch coverage is 12.00000% with 22 lines in your changes missing coverage. Please review.

Project coverage is 79.03%. Comparing base (785bb1e) to head (bbafade).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/_utils/various.py	8.33%	21 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20607      +/-   ##
==========================================
+ Coverage   79.00%   79.03%   +0.03%     
==========================================
  Files        1566     1557       -9     
  Lines      221034   220550     -484     
  Branches     2510     2514       +4     
==========================================
- Hits       174623   174308     -315     
+ Misses      45837    45667     -170     
- Partials      574      575       +1     

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[kjgoodrick]

@akshayka @mscolnick

[alexander-beedie]

@kjgoodrick: FYI, needs a small update (some triple-quotes where you currently have single-quotes with multiline strings, I think) due to the requirement to support py3.9.

I'll try it out once fixed, but looks like it could be a handy/straightforward addition 🤔

[eitsupi]

Is there any reason why this is not implemented in Rust?

[ritchie46]

This should not be implemented in python.

[kjgoodrick]

@alexander-beedie Thanks for pointing this out, I have updated the code such that it is now python 3.9 compatible.

@eitsupi @ritchie46 Thank you both for the comments, I think it should be fairly straightforward to move the mermaid generation code to rust and update the show_graph logic to handle some of the logic that is currently in display_dot_graph. I will open a new PR when that is ready.