graphql: Generate frozen requirements

Local run on my machine does not work properly. This generates pkg-resources==0.0.0 and
probably some other irrelevant things.

So, instead, I used docker:

```
$ docker run -ti -v $PWD:/src -u root --entrypoint=bash python:3.9
```

In another shell:

```
$ docker exec --user 1000 \
  $container_name /src/scripts/generate-frozen-requirements swh-graphql
...
```
- $container_name is derived out of `docker ps`.
- uid 1000 is derived out of your `id $USER`

Related to T4464