This ticket adds src/tox.ini to advertise the sage doctester to developers familiar with tox.

Later other test environments, such as linters, could be added here - see metaticket #28936.

CC: @embray @videlec @dimpase @vbraun

Component: doctest framework

Author: Matthias Koeppe

Branch/Commit: 109da23

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/28987