[1.2.X] Fixed an invalid import in the GeoDjango 3D tests.

import os

import re

from django.utils.unittest import TestCase

from unittest import TestCase

from django.contrib.gis.db.models import Union, Extent3D

from django.contrib.gis.geos import GEOSGeometry, Point, Polygon

from django.contrib.gis.utils import LayerMapping, LayerMapError