[stable-2.14] 📦 Switch sdist build-system to pure setuptools (#80255) (#80261)

* [stable-2.14] 📦 Switch sdist build-system to pure setuptools (#80255)

This patch modifies the in-tree build backend to build sdists that swap
out pointers to it in the `pyproject.toml`'s `[build-system]` section.

The effect of this is that the first build from source (for example,
from a Git checkout) uses our PEP 517 in-tree build backend. But the
produced tarball has `build-backend` set to `setuptools.build_meta`
which is the native build backend of `setuptools`. So any following
builds from that sdist will skip using the in-tree build backend,
calling the setuptools' one.
The good news is that if the first build generated the manpages, they
will be included and won't go anywhere even though, a different build
system is in place.

Combined with #80253, this will make sure not to modify the current
source checkout on that first build.

Co-authored-by: Matt Clay <matt@mystile.com>
(cherry picked from commit 7097df3eed979446830fc579613ffb9b7e7c19bf)

Co-authored-by: Sviatoslav Sydorenko <webknjaz@redhat.com>

* Make PEP 517 mutation tests use pinned old setuptools (#80262)

* Pin setuptools to lowest supported @ PEP 517 test

This allows catching the behavior of builds under old setuptools.

* Stop invoking `setup.py install` in tests

This is not the part we care about since it involves dealing with the
external runtime dependencies rather than building our source
distribution.

(cherry picked from commit eebfd71a6da396b011e664604c9991543205d780)

---------

Co-authored-by: Sviatoslav Sydorenko <webknjaz@redhat.com>

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