[test]Update python version in pytest image (#18477)

Signed-off-by: Nzhuwenxing <wenxing.zhu@zilliz.com>

tests/docker/Dockerfile

@@ -9,7 +9,7 @@
 # is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 # or implied. See the License for the specific language governing permissions and limitations under the License.
 
-FROM python:3.6.8-jessie
+FROM python:3.8-buster
 
 RUN apt-get update && apt-get install -y jq