Improve method is_planar

[dcoudert]

Fixes #35152.

We improve the method to avoid working with embeddings when the input (di)graph has multiple edges or loops, or when the input digraph has pairs of opposite arcs. In these cases, the notion of embedding is not consistently defined so far as it mixes neighbors and incident edges.

We now allow to set positions whenever embeddings are not involved.

We also avoid modifying the input graph in method is_planar from planarity.pyx and reorder some operations.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[dcoudert]

ping

[tscrim]

LGTM, but could you make one small tweak to the error message outputs and put one extra space on the second line after the break; e.g.,

NotImplementedError: a really long
 doctest output line

Once done, you can set a positive review.

[dcoudert]

Thank you for the review.

[github-actions]

Documentation preview for this PR (built with commit 933707f; changes) is ready! 🎉