goal: Make the stretch-swh's build for the scheduler ok

Relevant commit:
```
cd swh-scheduler
git checkout debian/stretch-swh  # 15bc20e66eb2fa8fcd59a17d951c4fdb52b1d62a
```

```
$ grep hypothesis debian/control
               python3-hypothesis (>= 3.11.0~),
```

Relevant command:
```
gbp buildpackage --git-builder=sbuild -As --force-orig-source --extra-repository='deb http://ftp.debian.org/debian stretch-backports main' --extra-repository='deb [trusted=yes] https://debian.softwareheritage.org/ stretch-swh main' --extra-repository='deb [trusted=yes] http://apt.postgresql.org/pub/repos/apt/ stretch-pgdg main'
```

Relevant output from command (`apt-get update` acknowledges the extra repositories, not the dependency asked for):
```
...
Reading package lists...
Hit:1 http://ftp.debian.org/debian stretch-backports InRelease
Get:2 http://apt.postgresql.org/pub/repos/apt stretch-pgdg InRelease [46.3 kB]
Ign:3 https://debian.softwareheritage.org stretch-swh InRelease
Hit:4 https://debian.softwareheritage.org stretch-swh Release
Ign:5 https://debian.softwareheritage.org stretch-swh Release.gpg
Ign:2 http://apt.postgresql.org/pub/repos/apt stretch-pgdg InRelease
...
 sbuild-build-depends-main-dummy : Depends: python3-hypothesis (>= 3.11.0~) but 3.6.1-1 is to be installed
```

Relevant build (which i claim is ok):
```
https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DSCH/job/gbp-buildpackage/13/console
```

Related D1110