python安装opencv报错ERROR: Could not build wheels for opencv-python, which is required to install pyproj

python3.6安装opencv遇到报错。如下所示：
pip install opencv-python报错。

  Please check the install target is valid and see CMake's output for more information.
  ----------------------------------------
  ERROR: Failed building wheel for opencv-python
Failed to build opencv-python
ERROR: Could not build wheels for opencv-python, which is required to install pyproject.toml-based projects

原因是使用pip install opencv-python命令安装的是最新版本，python3.6不支持。所以找一个python3.6支持的版本。如opencv-python==4.3.0.38。

pip install -i https://pypi.douban.com/simple/ pip install opencv-python==4.3.0.38