[1.5.x] Fixed geos test to prevent random failure

Points in the test fixtures have 20 as max coordinate.
Backport of 87854b0 from master.

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

@@ -688,7 +688,7 @@ def test_mutable_geometries(self):
             for i in range(len(mp)):
                 # Creating a random point.
                 pnt = mp[i]
-                new = Point(random.randint(1, 100), random.randint(1, 100))
+                new = Point(random.randint(21, 100), random.randint(21, 100))
                 # Testing the assignment
                 mp[i] = new
                 s = str(new) # what was used for the assignment is still accessible