And fix a test failure in the sandbox due to /home
existing on Darwin but not being accessible in the sandbox since it's a
symlink to /System/Volumes/Data/home, see
https://github.com/NixOS/nix/actions/runs/4205378453/jobs/7297384658#step:6:2127:

    C++ exception with description "error: getting status of /home/schnitzel/darmstadt/pommes: Operation not permitted" thrown in the test body.

On Linux this wasn't a problem because there /home doesn't exist in the sandbox

Copies part of the tree from Pythonix at
Mic92/pythonix@fbc8490
into the ./python subdirectory.

These Python bindings don't build yet with the current Nix version,
which is why they're not built yet in this commit.

Mic92 confirmed that he's okay with the license being changed to the Nix
one: NixOS#7735 (comment)

Co-Authored-By: Jörg Thalheim <joerg@thalheim.io>

The python bindings initialised from Pythonix haven't been updated in
some Nix versions and would not compile anymore. This commit still
doesn't include a working python bindings build, but these are the
Nix-update-caused changes that will be necessary for that

Makes the python bindings build, both incrementally and for CI.
Documentation is not yet included

Using the configuration file from NixOS#6721 for less conflicts

Will be useful for documentation

And don't use the deprecated python3 meson module

Not sure why I added it originally

Mirroring what the main Nix build does

And test that null bytes currently end strings

It didn't cause a problem because it converted True to 1, which still
allowed 1 == True to succeed in Python (using assertEqual)

That in fact doesn't work for importing

Using a symlink hacky and breaks in some ways

So that we don't have to rebuild Nix every time the python bindings
change

- BASH_SOURCE[0] doesn't exist for evaluations under `eval`
- NIX_STORE may not be set

The former is deprecated

Previously the python bindings derivation would use Nix as its source,
call configure on that, just to extract the two files tests/init.sh and
(the generated one) tests/common/vars-and-functions.sh

The new approach is to instead have a separate derivation extracting
these two files and having a small wrapper around them

Tests that the Nix bindings can be used as a Python dependency in Nix
builds

Docs still needed

Creating a new one for each call is wasteful and doesn't scale
to a composable interface, as the same EvalState must be used for
operations on the same Values.

Ideally we'd store EvalState in the python module, or make it a
first class object.