(from #30410)

This is so that all of the tox environments (#30410, #30408) can be run on files and directories.

Example:

$ ./sage -startuptime src/sage/tensor/ src/sage/geometry/polyhedron/base.py

CC: @dimpase @tobiasdiez @fchapoton @jhpalmieri @tscrim @vbraun

Component: doctest framework

Author: Matthias Koeppe

Branch/Commit: 306c390

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/30452