This drops the pytest-postgresql from requirements-db.txt. This is not a
required dependency for the swh.core.db module itself.
It's a requirement for the swh.core.db.pytest_plugin. This module should become
its own module as a test requirements for other swh modules (including
swh.core.db but not limited to it).
So this commit adds a dedicated requirements-db-pytestplugin.txt file with such
dependency. This adds such dependency as a test dependency for the swh.core.db
module (as it's needed for tests of the module).
Related to T2746