test: fix ImportError if psycopg2 not installed

test_postgresql was reaching into roundup.backends.back_postgresql
before checking to see if psycopg2 was available. Move the (partly
duplicated) import inside the guard so it imports only if psycopg2 is
available.

Author: rouilj

test/test_postgresql.py

@@ -21,7 +21,6 @@
 
 from roundup.hyperdb import DatabaseError
 from roundup.backends import get_backend, have_backend
-from roundup.backends.back_postgresql import db_command, get_database_schema_names
 
 from .db_test_base import DBTest, ROTest, config, SchemaTest, ClassicInitTest
 from .db_test_base import ConcurrentDBTest, HTMLItemTest, FilterCacheTest
@@ -36,7 +35,8 @@
         reason='Skipping PostgreSQL tests: backend not available'))
 else:
     try:
-        from roundup.backends.back_postgresql import psycopg2, db_command
+        from roundup.backends.back_postgresql import psycopg2, db_command,\
+            get_database_schema_names
         db_command(config, 'select 1')
         skip_postgresql = lambda func, *args, **kwargs: func
     except( DatabaseError ) as msg: