Extend PATH to make xzcat available.

Extend PATH to make xzcat available.

Enforce python3 usage, as the newer version requires python3.

After the changes in 565f4986, Flask need recent version of MarkupSafe
so we must align the version of MarkupSafe and Flask.

1c4a3c69 (stack/slapos.cfg: version up eggs (requests 2.31.0 and
friends), 2023-10-04) broke component/nxdtest/test-py2.cfg and
others using pytest on python2

This part of 30804165 (component/apache-php: add missing xz in PATH,
2023-10-17) was pushed accidentally

This is required for xz archives

See merge request !1450

This is required because it depends on an older version of Jinja and
updating in 565f4986 (stack/slapos: version up Flask and friends
(itsdangerous, Jinja2, MarkupSafe, Werkzeug), 2023-10-12) broke jupyter
software

At the same time, move some pins from stack/erp5 to stack/slapos as they
become used by more softwares

previous patch - 24f5a96c (software/slapos-sr-testing: disallow usage
of system python3, 2023-10-02) - was incorrect, an `entry-point` is
required for a script to be generated.