Use an import style reported not broken on Python 3.6

Couldn't reproduce the issue but it has been documented in #550.

NEWS

@@ -20,6 +20,8 @@ What's new in psycopg 2.7.2
   (:ticket:`#545`).
 - Fixed `~psycopg2.extras.ReplicationCursor.consume_stream()`
   *keepalive_interval* argument (:ticket:`#547`).
+- Maybe fixed random import error on Python 3.6 in multiprocess
+  environment (:ticket:`#550`).
 - Fixed random `!SystemError` upon receiving abort signal (:ticket:`#551`).
 - Parse intervals returned as microseconds from Redshift (:ticket:`#558`).
 - Added `~psycopg2.extras.Json` `!prepare()` method to consider connection

lib/__init__.py

@@ -65,7 +65,7 @@ from psycopg2 import tz                             # noqa
 
 # Register default adapters.
 
-import psycopg2.extensions as _ext
+from psycopg2 import extensions as _ext
 _ext.register_adapter(tuple, _ext.SQL_IN)
 _ext.register_adapter(type(None), _ext.NoneAdapter)
 

lib/pool.py

@@ -25,7 +25,7 @@ This module implements thread-safe (and not) connection pools.
 # License for more details.
 
 import psycopg2
-import psycopg2.extensions as _ext
+from psycopg2 import extensions as _ext
 
 
 class PoolError(psycopg2.Error):

lib/psycopg1.py

@@ -28,12 +28,12 @@ old code while porting to psycopg 2. Import it as follows::
 # FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 # License for more details.
 
-import psycopg2._psycopg as _2psycopg   # noqa
+from psycopg2 import _psycopg as _2psycopg   # noqa
 from psycopg2.extensions import cursor as _2cursor
 from psycopg2.extensions import connection as _2connection
 
 from psycopg2 import *      # noqa
-import psycopg2.extensions as _ext
+from psycopg2 import extensions as _ext
 _2connect = connect