db.pytest_plugin: Make pytest-postgresql an optional runtime dependency

Currently, this pulls the postgresql installation when installing swh.core which is not something wanted. That bypasses the current production pinning done in puppet and installs the latest postgresql-13 for example.

That particular dependency should be installed alongside swh modules which needs the core db pytest-plugin for their tests.

As impacts, i expected current packages needing this to break a bit but those already declares the pytest-postgresql as test requirements (i probably forgot to remove them). So, nothing to do, their builds will stay green:

• swh.scheduler
• swh.storage
• swh.vault

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

Test Plan

tox

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Migrated from D4376 (view on Phabricator)