sage.graphs: Do not use SAGE_TMP in doctests

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(split out from #33213)

CC:  @orlitzky @dcoudert @dimpase @fchapoton

Component: **refactoring**

Author: **Michael Orlitzky**

Branch/Commit: **[`976f0d6`](https://github.com/sagemath/sagetrac-mirror/commit/976f0d686ee5375ed3894009a13b2cd54fcd89f2)**

Reviewer: **David Coudert, Dima Pasechnik**

_Issue created by migration from https://trac.sagemath.org/ticket/33829_

