gh-40987: Cache is_projective_planar() method for graphs
The main example of this (the Petersen graph) is taking over six minutes
on the CI, and even longer on my machine. We already know the answer
when the graph is constructed, so by making it a `@cached_method`, the
method becomes effectively instantaneous.
We still want to test the computation, but that can be done via pytest
(and now is) rather than in a user-facing example.
URL: https://github.com/sagemath/sage/pull/40987
Reported by: Michael Orlitzky
Reviewer(s): David Coudert
