As the types-beautifulsoup4 package gets installed in the swh virtualenv
as it is a swh-scanner test dependency, some mypy errors were reported
related to beautifulsoup4 typing.
As the returned type for the find method of bs4 is the following union:
Tag | NavigableString | None, isinstance calls must be used to ensure
proper typing which is not great.
So prefer to use the select_one method instead where a simple None check
must be done to ensure typing is correct as it is returning Optional[Tag].
In a similar manner, replace use of find_all method by select method.
It also has the advantage to simplify the code.
Ensure that all lister classes have the same set of mandatory parameters
in their constructors, notably: scheduler, url, instance and credentials.
Add a new test checking listers classes have mandatory parameters declared
in their constructors. The purpose is to avoid deployment issues on staging
or production environment as celery tasks can fail to be executed if mandatory
parameters are not handled by listers.
Reated to swh/infra/sysadm-environment#5030.
