Add a minimal pytest configuration, so that calling pytest from src passes without any errors (and without finding any tests for the moment).

This is a preparation for future usages of pytest, such as #30362 or #30738, and is in the general spirit of #28936 (since pytest is perhaps the testing framework for python). For example, one could imagine using pytest doctest_modules to run all sage doctests as well.

CC: @mkoeppe

Component: build

Author: Tobias Diez

Branch/Commit: 462f071

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/31003