Refactor of trytond.config to add cache

This removes the need to store configuration in global variable and so make it
easier to update for example for testing.

Closes #14249

trytond-gis/trytond_gis/__init__.py

@@ -1,6 +1,6 @@
 # This file is part of trytond_gis.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
-from trytond.config import config
+import trytond.config as config
 
 from .const import WGS_84
 

trytond-gis/trytond_gis/postgis/table.py

@@ -2,8 +2,8 @@
 # this repository contains the full copyright notices and license terms.
 import logging
 
+import trytond.config as config
 from trytond.backend.postgresql.table import TableHandler as PGTableHandler
-from trytond.config import config
 from trytond.transaction import Transaction
 from trytond_gis.const import GIS_SQL_TYPE_RE, WGS_84