python3-libfdt: package is only building for one specific python version

[Dan Bungert]

Imported from Debian bug http://bugs.debian.org/1015271:

Package: python3-libfdt
Version: 1.6.1-2+b1
Severity: important
X-Debbugs-Cc: <email address hidden>

Control: -1 blocks 1012369

Dear Maintainer,

The python3-libfdt package is currently only built against the
python3-dev Build-Dependency:

```
$ ls /usr/lib/python3/dist-packages/_libfdt*
/usr/lib/python3/dist-packages/_libfdt.cpython-310-x86_64-linux-gnu.so
```

This means that python3-libfdt is currently:

- Available for python3.10
- Not available for python3.9

Most of the time we have two concurrent Python versions in Debian to
allow for smooth upgrades. This is why this package is no exception and
needs to be built for every supported Python version in Debian.

I have tried to provide a patch using `py3versions -s` and replacing
`python3-dev` with `python3-all-dev` without success. It seems that I am
not able to pass multiple Python versions to the Makefile.

The patch is attached as a starting point, but I will not mark the bug
with the patch attribute since this patch is not working and needs
refinement.

I think folks on #debian-python on OFTC will be happy to help if you
encounter the same struggle as I did and need to reach out.

Thanks in advance.

Agathe