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 opens the db-pytestplugin key within the setup.py and then
references that test dependency for the swh.core.db modules (as it's needed for
tests of the module).

Related to T2746

requirements-db-pytestplugin.txt

+# requirements for swh.core.db.pytest_plugin
+pytest-postgresql

requirements-db.txt

 # requirements for swh.core.db
 psycopg2
 typing-extensions
-pytest-postgresql

setup.py

@@ -55,11 +55,13 @@ setup(
     extras_require={
         "testing-core": parse_requirements("test"),
         "logging": parse_requirements("logging"),
-        "db": parse_requirements("db"),
+        "db": parse_requirements("db", "db-pytestplugin"),
         "testing-db": parse_requirements("test-db"),
         "http": parse_requirements("http"),
         # kitchen sink, please do not use
-        "testing": parse_requirements("test", "test-db", "db", "http", "logging"),
+        "testing": parse_requirements(
+            "test", "test-db", "db", "db-pytestplugin", "http", "logging"
+        ),
     },
     include_package_data=True,
     entry_points="""