drop python3-keyring from Recommends to Suggests

We should drop python3-keyring from Recommnds to Suggests. Having python3-keyring
as Recommends means it will always be installed when python-launchpadlib is
installed unless --no-install-recommends is specified. Many headless systems,
such as ubuntu-server, may require python-launchpadlib; but as far as I can
tell, there is no/limited need for a keyring service on a headless system.

The maintainers of python-launchpadlib have previously stated that
python3-keyring is not a hard dependency of python-launchpadlib and dropped
it from Depends to Suggests explicitly to avoid including unnecessary packages
on non-desktop machines[1]. There is a mismatch between the control file,
which moved python3-keyring to Recommends, and the changelog, which stated
Suggests. The wording in the changelog was updated to "Drop from Depends to
Recommends" in a later commit[2] (not currently in the imported source tree),
however if the motivation for the change is correct (which I believe it is),
then [2] should have moved python3-keyring from Recommends to Suggests.

A delta was previously introduced in python-secretstorage[3] to stop these
larger dependency packages from automatically being installed on server/cloud
images, however we should move the change here to python-launchpadlib as I
think it makes more sense semantically. The author of python-secretstorage has
also raised their concerns[4] with the delta we've introduced in that package,
which points out we are making python-secretstorage useless, and therefore the
dependent functionality in python3-keyring always useless (unless a Secret
Service package is already installed on the system).

I have already submitted a bug report[5] on Debian BTS to discuss this change
upstream, as well as a merge request[6] on the upstream VCS.

I will attach a debdiff with my proposed change.

[1] https://salsa.debian.org/python-team/packages/python-launchpadlib/-/commit/842dd5191ca3ef1acb800579b7a02cda6cbeeb31

[2] https://salsa.debian.org/python-team/packages/python-launchpadlib/-/commit/9ec5e3b0278f4c43422174a3a77d4075a88d7103

[3] http://changelogs.ubuntu.com/changelogs/pool/main/p/python-secretstorage/python-secretstorage_3.3.3-1ubuntu1/changelog

[4] https://bugs.launchpad.net/ubuntu/+source/python-secretstorage/+bug/2041695/comments/1

[5] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1056553

[6] https://salsa.debian.org/python-team/packages/python-launchpadlib/-/merge_requests/4