setup: Separate pytest-postgresql dependency and declare it when needed

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 swh/infra/sysadm-environment#2746 (closed)

Test Plan

tox

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Migrated from D4543 (view on Phabricator)