Fix switching between --with-system-python3 and --without-system-python3

[mkoeppe]

As noted in build/make/deps (and reported in #27824):

# TODO: If we reconfigure to build our own Python after having used the system
# Python, files installed to create the virtualenv should be *removed*.  That
# could either be done here by the makefile, or in an spkg-preinst for python3

In particular, the symlinks installed by venv together with the flawed copying from DESTDIR (see #29585) can cause the system python to be overwritten (observed on macOS).

#30687 solves this problem using an spkg-legacy-uninstall script.

In this ticket, we develop a cleaner solution by setting up the venv into a script package _venv. Together with #29386 (Install script packages via sage-spkg), this will take care of uninstalling the symlinks before writing the python3 installation to SAGE_LOCAL.

Depends on #29386
Depends on #30687

CC: @mezzarobba @dimpase

Component: packages: standard

Issue created by migration from https://trac.sagemath.org/ticket/29708

[mkoeppe]

Dependencies: #29386

[mkoeppe]

Changed dependencies from #29386 to #29386, #30687

[mkoeppe]

comment:5

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.