[1.7.x] Fixed #22456 -- Replaced 900913 EPSG code by 3857 in some tests

3857 is now the official EPSG code for the Google spherical mercator
projection and some recent versions of GDAL do not recognize 900913
any longer.
Thanks Tim Graham for the report and initial patch.
Backport of c082f3c74c from master.

django/contrib/gis/geos/tests/test_geos.py

@@ -1005,7 +1005,7 @@ class GEOSTest(unittest.TestCase, TestDataMixin):
         tgeoms = get_geoms(self.geometries.points)
         tgeoms.extend(get_geoms(self.geometries.multilinestrings, 4326))
         tgeoms.extend(get_geoms(self.geometries.polygons, 3084))
-        tgeoms.extend(get_geoms(self.geometries.multipolygons, 900913))
+        tgeoms.extend(get_geoms(self.geometries.multipolygons, 3857))
 
         # The SRID won't be exported in GEOS 3.0 release candidates.
         no_srid = self.null_srid == -1

django/contrib/gis/tests/test_spatialrefsys.py

@@ -110,11 +110,11 @@ class SpatialRefSysTest(unittest.TestCase):
         """
         from django.contrib.gis.utils import add_srs_entry
 
-        add_srs_entry(900913)
+        add_srs_entry(3857)
         self.assertTrue(
-            SpatialRefSys.objects.filter(srid=900913).exists()
+            SpatialRefSys.objects.filter(srid=3857).exists()
         )
-        srs = SpatialRefSys.objects.get(srid=900913)
+        srs = SpatialRefSys.objects.get(srid=3857)
         self.assertTrue(
             SpatialRefSys.get_spheroid(srs.wkt).startswith('SPHEROID[')
         )

django/contrib/gis/utils/srs.py

@@ -9,12 +9,10 @@ def add_srs_entry(srs, auth_name='EPSG', auth_srid=None, ref_sys_name=None,
     to the `spatial_ref_sys` table of the spatial backend.  Doing this enables
     database-level spatial transformations for the backend.  Thus, this utility
     is useful for adding spatial reference systems not included by default with
-    the backend -- for example, the so-called "Google Maps Mercator Projection"
-    is excluded in PostGIS 1.3 and below, and the following adds it to the
-    `spatial_ref_sys` table:
+    the backend:
 
     >>> from django.contrib.gis.utils import add_srs_entry
-    >>> add_srs_entry(900913)
+    >>> add_srs_entry(3857)
 
     Keyword Arguments:
      auth_name: