update SQL parameter quoting syntax per mapnik/mapnik#3618

Author: artemp

test/python_tests/postgis_test.py

@@ -1248,7 +1248,7 @@ def test_handling_of_discarded_key_field():
 
     def test_variable_in_subquery1():
         ds = mapnik.PostGIS(dbname=MAPNIK_TEST_DBNAME, table='''
-           (select * from test where @zoom = 30 ) as tmp''',
+           (select * from test where !@zoom! = 30 ) as tmp''',
                             geometry_field='geom', srid=4326,
                             autodetect_key_field=True)
         fs = ds.featureset(variables={'zoom': 30})