Use beautifulsoup4 CSS selectors to simplify code and type checking

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.

swh/lister/rubygems/lister.py

@@ -1,4 +1,4 @@
-# Copyright (C) 2022-2023  The Software Heritage developers
+# Copyright (C) 2022-2024  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -82,8 +82,8 @@ class RubyGemsLister(StatelessLister[RubyGemsListerPage]):
     def get_latest_dump_file(self) -> str:
         response = self.http_request(self.RUBY_GEMS_POSTGRES_DUMP_LIST_URL)
         xml = BeautifulSoup(response.content, "xml")
-        contents = xml.find_all("Contents")
-        return contents[-1].find("Key").text
+        contents = xml.select("Contents")
+        return contents[-1].select("Key")[0].text
 
     def create_rubygems_db(
         self, postgresql: Postgresql