.github/workflows/tox-gcc_spkg.yml: Remove python2

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Follow-up from #29669

Depends on #29669

CC:  @dimpase @jhpalmieri @kliem

Component: **porting**

Author: **Matthias Koeppe**

Branch/Commit: **[`eac7eca`](https://github.com/sagemath/sagetrac-mirror/commit/eac7eca8ffd825e248c50eb7c9a69a723d47642c)**

Reviewer: **Jonathan Kliem**

_Issue created by migration from https://trac.sagemath.org/ticket/29757_

