(from #30578)

We add a tox environment develop to src/tox.ini, enabling the command ../sage -tox -e develop for building and testing sagelib in editable (develop) mode in a venv.

Depends on #30578
Depends on #31377

CC: @tobiasdiez

Component: build

Keywords: sd111

Issue created by migration from https://trac.sagemath.org/ticket/30911