pip install opencv-python gives error on Raspbian

I'm using a Raspberry Pi 3 B+ with Raspbian installed. When I run pip install opencv-python I get the following message:

Looking in indexes: https://pypi.org/simple , https://www.piwheels.org/simple 
Collecting opencv-python
ERROR: Could not find a version that satisfies the requirement opencv-python (from version: none)
ERROR: No matching distribution found for opencv-python

I've tried pip install opencv-contrib-python, python3 -m pip install opencv-python and that gives me the same error. And pip install --upgrade pip before trying to install opencv, but it says I already have the latest version. What I have:

$ python --version
Python 2.7.13

 $ python3 --version
Python 3.6.3

 $ pip --version
pip 19.1 from /usr/local/lib/python3.6/site-packages/pip (python 3.6)