How to install libavformat.so.56 along with libavformat.so.57?

问题

I used pip to install opencv3 with the following command on an Anaconda virtual environment.

install opencv-python

I successfully installed it because I can see the package in pip list.

(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ pip list | grep opencv
opencv-python          3.4.0.14

However, when I do import cv2, I get the following error:

(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ python
Python 3.4.3 |Continuum Analytics, Inc.| (default, Aug 21 2015, 00:53:08) 
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/pi/.conda/envs/olfatf/lib/python3.4/site-packages/cv2/__init__.py", line 3, in <module>
    from .cv2 import *
ImportError: libavformat.so.56: cannot open shared object file: No such file or directory

I checked the /usr/lib/arm-linux-gnueabihf directory and I found:

(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ ls | grep libavformat
libavformat.a
libavformat.so
libavformat.so.57
libavformat.so.57.56.101

I you can see there, I have the version 57 but the version 56 is missing. I did research online and people suggested that I should install libavformat-dev

(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ sudo apt-get install libavformat-dev
Reading package lists... Done
Building dependency tree       
Reading state information... Done
libavformat-dev is already the newest version (7:3.2.10-1~deb9u1+rpt1).

But it says that the package already exists. I also tried installing ffmpeg.

(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ sudo apt-get install ffmpeg
Reading package lists... Done
Building dependency tree       
Reading state information... Done
ffmpeg is already the newest version (7:3.2.10-1~deb9u1+rpt1).

But it says that the package already exists.

How to get the 56 version? I use Raspberry Pi Strech