Gramps does not check [for] installed DB backend on DB selector

Fixes #10148

gramps/plugins/db/dbapi/postgresql.gpr.py

@@ -21,7 +21,14 @@ from gramps.gen.plug._pluginreg import register, STABLE, DATABASE
 from gramps.gen.const import GRAMPS_LOCALE as glocale
 _ = glocale.translation.gettext
 
-register(DATABASE,
+try:
+    import psycopg2
+    available = True
+except (ImportError, ValueError):
+    available = False
+
+if available:
+    register(DATABASE,
              id='postgresql',
              name=_('PostgreSQL'),
              name_accell=_('_PostgreSQL Database'),
@@ -33,4 +40,4 @@ register(DATABASE,
              databaseclass='PostgreSQL',
              authors=['Doug Blank'],
              authors_email=['doug.blank@gmail.com']
-)
+    )