When using #!/usr/bin/python3, we end up using Debian's system-wide
python rather than the copy shipped in the docker container. This
pollutes the generated virtualenv with the wheels generated by Debian
packaging, rather than the expected upstream wheels.

The appVersion will be a main and unique swh application set version
referencing all application deployment changes happening at one point in
time (one commit too).

Refs. sysadm-environment#4724

```
$ app_manager tag --help
Usage: app_manager.py tag [OPTIONS] COMMAND [ARGS]...

  Manipulate application tag, for example either determine the last tag or
  compute the next one. Without any parameters this lists the current
  application tags.

Options:
  --help  Show this message and exit.

Commands:
  latest
  list
  next
```

This will allow to:
- retrieve the last known tag for an application
- determine the next tag for an application (be it known or not)
- current list of tags of an application

This will raise when:
- providing both --next-tag and --last-tag for an application
- listing tags for an unknown application
- retrieving the last tag on an unknown application

Refs. sysadm-environment#4724

The hook was not installed in my repository.

Refs. sysadm-environment#4724

Ref. sysadm-environment#4724

This reads tags input from swh-apps repository and adapts accordingly the targetted
values.yaml.

Ref. sysadm-environment#4724

At the moment 2:
- generate-frozen-requirements
- list (with/without parameters)

This will ease reusing the same docker image for multiple calls to subcommands.

Ref. sysadm-environment#4724

The script opens whatever apps has the module name [1] in its current frozen
requirements. If the constraint of module name and version does not match, it outputs
said application as being impacted.

Example calls:
```
./scripts/list-apps-to-rebuild swh-loader-core 5.2.0
./scripts/list-apps-to-rebuild swh-loader-core v5.2.0
```

Ref. sysadm-environment#4724

This allows to build the container that will call the generate-frozen-requirements
script.

That will be usable from ci or other sysadm.

Use:
```
cd swh-apps/scripts
docker build -t build-deps .
cd ../../swh-apps
docker run -v $PWD:/src --user 1000 build-deps $APP
```

This allows better caching of modules with no pyproject.toml